International Orthopaedics (SICOT) DOI 10.1007/s00264-016-3145-z
ORIGINAL PAPER
How to best measure femoral length and lateralisation after total hip arthroplasty on antero-posterior pelvic radiographs Nicolas Bonin 1 & Laurent Jacquot 2 & Laurent Boulard 3 & Patrick Reynaud 4 & Mo Saffarini 5 & Sébastien Lustig 6
Received: 20 November 2015 / Accepted: 16 February 2016 # SICOT aisbl 2016
Abstract Purpose Various methods exist for measuring limb length and lateralisation after total hip arthroplasty. Most of them utilise standard anteroposterior (AP) pelvic radiographs, but their results can be affected by patient position during imaging and thus the position of the lower limb on the coronal plane. The aim of this study is to evaluate how commonly used measuring methods of limb lengthening and femoral offset are affected by the position of the lower limb in the coronal plane. Methods A standing pelvic AP radiograph post implantation of a right total hip prosthesis was digitised. The right femur and its femoral stem were digitally segmented, such that they could be positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction, with respect to the centre of rotation. Various limb lengths and implant lateralisation were also digitised. We obtained nine x-rays differing one to three variables. Twice four independent surgeons performed three femoral length measurement
methods and femoral offset measurement methods. Intra and inter-observer error as well as the effect of the femoral position on the measurements were studied. Results With respect to length measurements, the distance between the centre of rotation (C) and the tip of the lesser trochanter (LT) increased by 3 mm per cm of lateralisation. This measurement was not affected by the hip position in abduction or adduction. The distance between the tip of the lesser or greater trochanter (GT) and the horizontal passing through the centre of rotation was strongly affected by the hip position in abduction or adduction. With respect to offset, the distance between the centre of rotation and the greater trochanter (C-GT) was the most consistent and was not affected by variations in lengths or femoral axis. At the level of the lesser trochanter, the distance of the femoral anatomical axis and to Perkin’s line was heavily influenced by the femoral position. Conclusion The C-LT distance was consistent in measuring limb length and the C-GT distance was reliable in determining femoral offset regardless of the relative position of the femur.
* Nicolas Bonin [email protected]
Keywords Hip . Lateralisation . Length . Offset . Total hip arthroplasty . Pelvic radiographs
1
Lyon-Ortho-Clinic, 29B av. des sources, 69009 Lyon, France
2
Clinique d′Argonay, 685 route de Menthonnex, 74370 Argonnay, France
3
Polyclinique de la ligne Bleue, 9 avenue Rose Poirier, 88000 Epinal, France
4
Clinique E. de Vialar, 116 rue A. Charrial, 69003 Lyon, France
5
Accelerate Innovation Management SA, 1 rue de la Navigatino, 1201 Geneva, Switzerland
6
Centre Albert TRILLAT, 103 Grande rue de la Croix Rousse, 69004 Lyon, France
Introduction Various methods exist for measuring limb length and lateralisation after total hip arthroplasty (THA), most of which use standard antero-posterior (AP) pelvic radiographs [1–8]. Various anatomic landmarks and reference points have been described on those radiographs to obtain a number of measurements. Having the centres of rotation of both hips, the changes in limb length can be deduced from the distances between the centre of rotation and the tip of the lesser
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trochanter [1]. However the femoral lateralisation may alter this measurement. Limb length is more often measured by the distance between the tip of the lesser [9] or the greater trochanter [1, 10] and a horizontal line tangent to the inferior borders of both Bteardrops^. The results of this measurement can be influenced by the position of the limb being not strictly perpendicular to the pelvis horizontal reference during X-ray. Measurement of the femoral offset can be obtained by tracing a line parallel to the inter-teardrop line at the level of the lesser trochanter. On this line, the distance between the middle of the metaphysis and the vertical passing through the centre of rotation can be measured [10]. Similarly, the relative position of the femur in abduction or in adduction on the coronal plane may affect this measurement. The distance between the centre of rotation and the tip of the greater trochanter can also be measured, but the leg length may influence this measurement. While plain pelvic radiographs remains the standard to perform length and lateralisation’s femoral measurements, their accurate measure is subject to substantial error, if the individual pelvis and femur orientation with respect to the X-ray plate is not taken into consideration [11–13]. It is thus of special interest to control improved methods to accurately measure the post-operative femoral length and offset for a reasonable follow-up and the basis for any error analysis after THA. The aim of this study is to highlight the most accurate and reproducible method to measure limb length and femoral offset on AP pelvic X-ray radiographs post THA implantation by comparing variations in the results of different known measurement methods made in the same standard AP pelvic X-ray where femoral length, femoral offset and limb abduction or adduction were purposefully and precisely introduced.
Materials and methods AP pelvic X-ray radiographs post THA implantation remodelling (Fig. 1)
A standing pelvic AP radiograph post implantation of a right total hip prosthesis was digitised (Fig. 1a). An image editing software (Adobe Photoshop Element 2.0, San Jose, CA) was used to digitally segment the right femur and its femoral stem to the pelvis (Fig. 1b), such that they could be positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction with respect to the centre of rotation (Fig. 1c). Three different images of the same implant (identical length and offset) were therefore obtained (Fig. 2). This same implant was then digitally lengthened (Fig. 1d) giving three more images of a longer implant with the same offset and three positions, so-called pure lengthening
(Fig. 3). It was also lateralised with the same technique (Fig. 1e) giving three images of an implant of the same length with a larger offset and three positions, so-called pure lateralisation (Fig. 4). In total, we obtained nine images, which were randomly numbered (Table 1). The centres of rotation were traced in order to eliminate measurement biases. The contra-lateral hip was hidden. Femoral length measurement Three femoral length measurement methods were performed (Fig. 5): – – –
The distance between the centre of rotation and the tip of the lesser trochanter [1] (C-LT). The distance between the tip of the lesser trochanter and the horizontal passing through the centre of rotation [9] (HC-LT). The distance between the tip of the greater trochanter and the horizontal passing through the centre of rotation [1, 10] (HC-GT).
Femoral offset measurement Three femoral offset measurement methods were performed (Fig. 5): – –
–
The distance between the centre of rotation and the greater trochanter (C-GT), The distance, at the level of the lesser trochanter, between the middle of the metaphysis and the vertical (perpendicular to the teardrop line) passing through the centre of rotation [10] (VC-LT), The distance between the centre of rotation and the femoral anatomical axis [7, 14] (C-FA).
Twice four independent surgeons, who were kept unaware of the radiographs modifications and the study objectives, performed all measurements. Measurement analysis Two groups were assembled for the analysis of the results: –
–
One group of invariable length; where only the femoral position and implant lateralisation were changed (six different radiographs). In this group, variations in length measurements would correspond to biases induced by the measurement technique that was used. One group of invariable lateralisation; where only the femoral position and implant length were changed (six
International Orthopaedics (SICOT) Fig. 1 a Digitalisation of AP pelvic radiographs post implantation of a right THA. b image editing software used to digitally segment the right femur and its femoral stem to the pelvis. c femur and its femoral stem, digitally positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction. d femoral stem, digitally lengthened of 1 cm. e femoral stem, digitally lateralised of 1 cm
different radiographs). In this group, variations in femoral lateralisation measurements would correspond to biases induced by the measurement technique that was used.
excellent (0.81–1.00). Analysis of quantitative data was performed using non-parametric test (Mann–Whitney) with statistical significance accepted at p < 0.05.
Statistical analysis
Results
All analyses were performed with Microsoft Excel® 2008 and StatEl software (ad Science Society, Paris). We analysed Intra and inter-observer’s agreement using Kendall test. Kendall’s correlation coefficient was characterised as poor (0.00–0.20), fair (0.21–0.40), moderate (0.41–0.60), good (0.61–0.80) or
Group of invariable length
Fig. 2 Three different images of the same implant (identical length and offset) a right femur and its femoral stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
Intra and inter-observer discrepancies assessments on the length measurement showed that the measurement of distances C-LT, HC-LT and HC-GT are reproducible with
International Orthopaedics (SICOT) Fig. 3 Three different images of the same implant digitally lengthened (identical offset) a right femur and its femoral lengthen stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
excellent correlation (Kappa = 0.81, 0.87, 0.90 respectively). Maximum differences were less than 3 mm. Measurements results are summarised in Table 2. Within the invariable length group, modifying the femur position in adduction and abduction as well as offset augmentation significantly influenced the length measurements (Table 3): –
–
–
The C-LT value was significantly increased by an average of 3 mm per cm of lateralisation. However, this measurement was not affected by the femoral position in adduction or abduction (p = 0.09). The HC-LT value vas not influenced by lateralisation of the femur. However this measurement was significantly modified by femoral position (p < 0.001), with augmentation in adduction, and reduction in abduction, up to 4 mm with the use of a standard stem and up to 8 mm in case of 10 mm of lateralisation. The HC-GT value was the most sensitive to the variations of femoral position in adduction and abduction, with measurement deviations greater than 10 mm (p < 0.001). This value was also significantly influenced by femoral lateralisation.
Fig. 4 Three different images of the same implant digitally lateralised (identical length) a right femur and its femoral lateralised stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
Group of invariable lateralisation Intra and inter-observer discrepancies assessments on the lateralisation measurement showed that the measurement of distances C-GT and VC-LT are reproducible with excellent correlation (Kappa = 0.88, 0.88 respectively) and maximum differences of 5 mm. Measurement of dist a n c e s C - FA w e r e f o u n d p o o r l y r e p r o d u c i b l e (Kappa = 0.20) (Table 2). Within the invariable lateralisation group, modifying the femoral position in adduction or in abduction as well as increasing the length significantly influenced some of the offset measurements (Table 4): – –
–
The C-GT value was reproducible and was not altered by length or femoral axis variations (p = 0.06). The VC-LT value was not influenced by length variations (p = 0.2), but an adducted femoral axis decreased significantly the offset measurement by 10 mm, and an abducted femoral axis increased it by 10 mm (p < 0.001). The C-FA value was not influenced by the various test configurations (p = 0.3) but still varies within 8 millimetres according to the intra and inter-observer discrepancies (Kappa = 0.20).
International Orthopaedics (SICOT) Table 1 Nine pelvic x-rays post THA implantation differing one to three variables
Table 2 Intra and inter-observer measurements variations with respect to the measurement method
Modification
Lateralisation
Length measurements
C-LT
HC-LT
HC-GT
Identical length and offset
Pure lengthenning
Pure lateralisation
Angle
Length
90°
0
Standard
Δ intra-observer
0.6 (0.6) 2
0.8 (0.8) 3
0.6 (0.6) 2
Adduction 10°
0
Standard
Δ inter-observer
2.1 (0,6) 3
2.2 (0.8) 3
1.9 (0.9) 3
Abduction 10° 90°
0 +10 mm
Standard Standard
Lateralisation measurements
C-GT
VC-LT
C-FA
Adduction 10°
+10 mm
Standard
Abduction 10° 90°
+10 mm 0
Standard Lateralised
Δ intra-observer Δ inter-observer
0.6 (0.6) 2 1.6 (0.6) 2
0.8 (0.7) 2 4 (0.6) 5
1.5 (1.5) 7 5.9 (1.3) 8
Adduction 10° Abduction 10°
0 0
Lateralised Lateralised
Discussion The present study did not attempt to assess the reliability of the landmarks used for the measurements, such as the identification of the tip of the lesser or greater trochanter. With the use of strictly identical radiographs, these landmarks are considered reliable, with inter-observer errors between 2 % and 7 %, and intra-observer between 1 % and 6 %, which corresponds approximately to 1 mm [1, 2]. Considering the measurements of the centre to the greater trochanter (C-GT) and centre to the lesser trochanter (C-LT), our results are consistent to previous studies with a excellent intra and inter-observer reliance. However, variations in the measurements reliability was founded poor in the C-FA measurement, with inter-observer errors of up to 8 mm. This probably reflects the poor accuracy in which the femoral anatomical axis can be identified. Moreover, a few tenths of degrees error on the axis measurement is bound to be amplified and will translate into
Average (Standard deviation) Maximal deviation Units = millimeters
millimetres on the measurements distant to the reference points. Among the available methods to measure leg length, the distance from the ischium line to the lesser trochanter was not tested. The ischium line is known to be highly influenced by changes in the pelvic rotation [15]. Conversely, the interteardrop line is not influenced by pelvic rotation. It is identified in several studies as the most reliable acetabular reference in the coronal plane [15, 16]. Clinically, how length variations are tolerated remains controversial. White and Dougall [2] did not find any correlation between leg length discrepancies at six months post op and HSS or SF 36 scores. For Konyves and Bannister [4], good leg length restoration is correlated to the OHS score. The preferred method found in the literature for leg length measurements after a hip prosthesis [2–4, 6, 8] was described and validated by Williamson and Reckling [9], who observed correlations close to those of lower limbs pangonograms. This technique measures the distance between the tip of the lesser trochanter and an inter-teardrop line (HC-LT). This measurement is reproducible and is only minimally altered by lateralisation. It is nevertheless modified by changes in the femur position in the coronal plan. This modification is further worsened by any increase in femoral lateralisation. Table 3 Average length measurements obtained with the various measurement methods, on invariable length images
Fig. 5 Femoral length and offset measurement methods. Purple = length measurement methods. Blue = offset measurement methods
Length measurements
C-LT
HC-LT
HC-GT
Standard stem— femur 90° Lateralised stem—femur 90° Standard stem—femur varus 10° Standard stem—femur valgus 10° Lateralised stem—femur varus 10°
57 (0.7) 60 (0.9) 58 (1.1) 57 (0.7) 59 (0.6)
56 (1.1) 55 (1.0) 57 (0.7) 52 (0.7) 57 (1.1)
−8 (0.4) −9 (0.6) 2 (1.2) −17 (0.6) 2 (0.9)
Lateralised stem—femur valgus 10° Max deviation
60 (0.5) 5
50 (0.6) 8
−19 (0.6) 24
Average (Standard deviation) Units = millimeters
International Orthopaedics (SICOT) Table 4 Average lateralisation measurements obtained with the various measurement methods, on invariable offset images Lateralisation measurements
C-GT
VC-LT
C-FA
Standard stem—femur 90°
54 (0.6)
44 (1.6)
45 (2.3)
Stem + 10 mm—Femur 90°
54 (0.7)
45 (1.7)
46 (1.4)
Standard stem—varus 10° Standard stem—valgus 10°
54 (0.7) 54 (0.6)
34 (2.0) 54 (1.4)
45 (2.6) 43 (1.9)
Stem + 10 mm—varus 10°
54 (0.4)
34 (1.7)
46 (2.2)
Stem + 10 mm—valgus 10° Max deviation
54 (0.6) 2
56 (1.7) 27
44 (2.4) 8
Average (Standard deviation) Units = millimeters
Another measurement method uses the distance between the tip of the greater trochanter and the horizontal passing through the centre of rotation [5, 10] (HC-GT). Just as the former, this measurement is only reliable on radiographs where there is no abduction or adduction of the femur. Any modification on the coronal plane alters the measurement by a delta (Δ) according to the formula: Δ = C-FA . sinα where the angle Balpha^ corresponds to the adduction angle. BAlpha^ angle is positive in adduction and negative in abduction (Fig. 6). The measurement is therefore considerably altered by the femoral angulation and offset, producing deviations up to 24 mm. Reducing these biases requires measurements on perfectly well taken radiographs. Nevertheless, pelvis deviations originating from the spine on a supine installation, or from leg length discrepancy on a standing radiograph, induce a lower limb deviation that is no longer perpendicular to the teardrop
Fig. 6 Measurement variations depending on femoral varus or valgus positioning of α angle to the pelvis horizontal reference. Length variation: ΔHC-GT = C-FA . sinα. Offset variation: Δ VC-LT = C-LT . sinα
line [11–13]. Such a deviation can produce errors when using pelvic-referenced measurements methods. The global offset is the sum of the femoral offset and the lateralisation of the centre of rotation. Charnley [17] was one of the first to describe how critical is the restoration of the femoral offset. Femoral offset’s influence on health-related quality of life after total hip replacement is no more controversial [7, 18]. Restoring femoral offset requires being perfectly planned pre-operatively because no reliable reference can be used to control it during the operation. The most widely used technique was published by Steinberg and Harris [7]. It corresponds to the distance between the femoral head centre and the femoral anatomical axis (C-FA). This measurement is not affected by the position of the femur. However, it remains poorly reproducible with kendall’s inter-observer correlation coefficient of 0.20 in our study, likely due to the lack of accuracy in identifying the femoral axis by standard AP x-rays. A computerised measurement involving automatic tracing of the femoral axis and its perpendiculars would probably improve its accuracy, but such techniques are still to be evaluated [8, 19–21]. Hausselle et al. [10] measured the offset with a technique to the VC-LT measurement in the present study. Since the reference is the pelvis, this measurement is reliable only on radiographs where the femoral anatomical axis is perpendicular to the pelvis horizontal. Placement of the femur in adduction or in abduction by an angle α will introduce an error of Δ according to the formula: Δ = C-LT . sinα (Fig. 6). As a result, this measurement is altered by the femoral position and also by the variation in the femoral length. In the present study, deviations for identical lateralisation can reach 27 mm. The most reliable results are obtained with the measurement of CGT in the evaluation of the femoral offset, with maximum deviations of only 2 mm. This simple measurement is not influenced by length or femoral axis variations. However, limitations in the reliability of the measurements from bi-dimensional data still persist [13]. If the radiological evaluation is performed on planar projections, any variation of the femoral stem anteversion alters the offset measurement proportionally to the cosine of the anteversion angle. A 20° anteversion underestimates femoral offset by 15 % [10]. For this reason, it is critical to measure on an X-ray where the femoral anteversion is compensated by proper limb rotation. When the offset of the implanted stem is known, it is possible to check if the prosthetic neck has been radiographed in a true frontal view, and if not, its anteversion α can be worked out with the formula: cosα = Xray measurement/stem offset. The goal is still to compare the operated limb offset and length to the contralateral side. Bony landmarks to visualise the trochanters are critical to screen out radiographs where both femurs would not appear in the same rotation. In those cases, comparative measurements cannot be achieved. With its three-dimensional abilities, tomodensitometry, associated
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to the relevant software, is the reference tool to measure lengths, lateralisations and rotations of implants [22, 23]. This study identifies the limitations of some measurement techniques with regards to their poor reliability and reproducibility and demonstrates that regardless of the method, some degree of inaccuracy persists. In conclusion, femoral length may not be possibly measured with accuracy greater than 5 mm on standard AP pelvic X-rays. It is best obtained from the distance between the centre of rotation and the tip of the lesser trochanter (C-LT), provided that femoral offset has not changed more than 10 mm. Femoral offset can be measured with an excellent accuracy of approximately 2 mm from the distance between the centre of rotation and the greater trochanter (C-GT), provided that the x-rays are acquired with a true frontal view of the femoral neck. Compliance with ethical standards Conflict of interest and funding No funds were received in support of this study. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
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9.
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12.
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References 1.
2. 3.
4.
5.
6.
7.
Eggli S, Pisan M, Muller ME (1998) The value of preoperative planning for total hip arthroplasty. J Bone Joint Surg (Br) 80(3): 382–390 White TO, Dougall TW (2002) Arthroplasty of the hip. Leg length is not important. J Bone Joint Surg (Br) 84(3):335–338 Della Valle AG, Padgett DE, Salvati EA (2005) Preoperative planning for primary total hip arthroplasty. J Am Acad Orthop Surg 13(7):455–462 Konyves A, Bannister GC (2005) The importance of leg length discrepancy after total hip arthroplasty. J Bone Joint Surg (Br) 87(2):155–157 Debarge R, Lustig S, Neyret P, Ait Si Selmi T (2008) Confrontation of the radiographic preoperative planning with the postoperative data for uncemented total hip arthroplasty. Rev Chir Orthop Reparatrice Appar Mot 94(4):368–375 Unnanuntana A, Wagner D, Goodman SB (2009) The accuracy of preoperative templating in cementless total hip arthroplasty. J Arthroplasty 24(2):180–186 Steinberg B, Harris, W (1992) The offset problem in total hip arthroplasty. Contemp Orthop 24:556
17. 18.
19.
20.
21.
22.
23.
Shaarani SR, McHugh G, Collins DA (2013) Accuracy of digital preoperative templating in 100 consecutive uncemented total hip arthroplasties: a single surgeon series. J Arthroplasty 28(2):331– 337 Williamson JA, Reckling FW (1978) Limb length discrepancy and related problems following total hip joint replacement. Clin Orthop Relat Res 134:135–138 Hausselle J, Moreau PE, Wessely L, de Thomasson E, Assi A, Parratte S, Essig J, Skalli W (2012) Intra- and extra-articular planes of reference for use in total hip arthroplasty: a preliminary study. Int Orthop 36(8):1567–1573 Craiovan B, Renkawitz T, Weber M, Grifka J, Nolte L, Zheng G (2014) Is the acetabular cup orientation after total hip arthroplasty on a two dimension or three dimension model accurate? Int Orthop 38(10):2009–2015 Kim YH, Cho KH, Park YG (2015) Is the acetabular cup orientation after total hip arthroplasty on a two-dimensional or three-dimensional model accurate? Int Orthop 39(4):819– 820 Tsai TY, Dimitriou D, Li G, Kwon YM (2014) Does total hip arthroplasty restore native hip anatomy? three-dimensional reconstruction analysis. Int Orthop 38(8):1577–1583 Schmidutz F, Beirer M, Weber P, Mazoochian F, Fottner A, Jansson V (2012) Biomechanical reconstruction of the hip: comparison between modular short-stem hip arthroplasty and conventional total hip arthroplasty. Int Orthop 36(7):1341–1347 Goodman SB, Adler SJ, Fyhrie DP, Schurman DJ (1988) The acetabular teardrop and its relevance to acetabular migration. Clin Orthop Relat Res 236:199–204 Meermans G, Malik A, Witt J, Haddad F (2011) Preoperative radiographic assessment of limb-length discrepancy in total hip arthroplasty. Clin Orthop Relat Res 469(6):1677–1682 Charnley J (1979) Low friction arthroplasty of the hip: theory and practice. Springer, New York Liebs TR, Nasser L, Herzberg W, Ruther W, Hassenpflug J (2014) The influence of femoral offset on health-related quality of life after total hip replacement. Bone Joint J 96-B(1):36–42 Gamble P, de Beer J, Petruccelli D, Winemaker M (2010) The accuracy of digital templating in uncemented total hip arthroplasty. J Arthroplasty 25(4):529–532 Kumar PG, Kirmani SJ, Humberg H, Kavarthapu V, Li P (2009) Reproducibility and accuracy of templating uncemented THA with digital radiographic and digital TraumaCad templating software. Orthopedics 32(11):815 The B, Verdonschot N, van Horn JR, van Ooijen PM, Diercks RL (2007) Digital versus analogue preoperative planning of total hip arthroplasties: a randomized clinical trial of 210 total hip arthroplasties. J Arthroplasty 22(6):866–870 Sariali E, Mouttet A, Pasquier G, Durante E, Catone Y (2009) Accuracy of reconstruction of the hip using computerised threedimensional pre-operative planning and a cementless modular neck. J Bone Joint Surg (Br) 91(3):333–340 Pasquier G, Ducharne G, Ali ES, Giraud F, Mouttet A, Durante E (2010) Total hip arthroplasty offset measurement: is C T scan the most accurate option? Orthop Traumatol Surg Res 96(4):367–375
ORIGINAL PAPER
How to best measure femoral length and lateralisation after total hip arthroplasty on antero-posterior pelvic radiographs Nicolas Bonin 1 & Laurent Jacquot 2 & Laurent Boulard 3 & Patrick Reynaud 4 & Mo Saffarini 5 & Sébastien Lustig 6
Received: 20 November 2015 / Accepted: 16 February 2016 # SICOT aisbl 2016
Abstract Purpose Various methods exist for measuring limb length and lateralisation after total hip arthroplasty. Most of them utilise standard anteroposterior (AP) pelvic radiographs, but their results can be affected by patient position during imaging and thus the position of the lower limb on the coronal plane. The aim of this study is to evaluate how commonly used measuring methods of limb lengthening and femoral offset are affected by the position of the lower limb in the coronal plane. Methods A standing pelvic AP radiograph post implantation of a right total hip prosthesis was digitised. The right femur and its femoral stem were digitally segmented, such that they could be positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction, with respect to the centre of rotation. Various limb lengths and implant lateralisation were also digitised. We obtained nine x-rays differing one to three variables. Twice four independent surgeons performed three femoral length measurement
methods and femoral offset measurement methods. Intra and inter-observer error as well as the effect of the femoral position on the measurements were studied. Results With respect to length measurements, the distance between the centre of rotation (C) and the tip of the lesser trochanter (LT) increased by 3 mm per cm of lateralisation. This measurement was not affected by the hip position in abduction or adduction. The distance between the tip of the lesser or greater trochanter (GT) and the horizontal passing through the centre of rotation was strongly affected by the hip position in abduction or adduction. With respect to offset, the distance between the centre of rotation and the greater trochanter (C-GT) was the most consistent and was not affected by variations in lengths or femoral axis. At the level of the lesser trochanter, the distance of the femoral anatomical axis and to Perkin’s line was heavily influenced by the femoral position. Conclusion The C-LT distance was consistent in measuring limb length and the C-GT distance was reliable in determining femoral offset regardless of the relative position of the femur.
* Nicolas Bonin [email protected]
Keywords Hip . Lateralisation . Length . Offset . Total hip arthroplasty . Pelvic radiographs
1
Lyon-Ortho-Clinic, 29B av. des sources, 69009 Lyon, France
2
Clinique d′Argonay, 685 route de Menthonnex, 74370 Argonnay, France
3
Polyclinique de la ligne Bleue, 9 avenue Rose Poirier, 88000 Epinal, France
4
Clinique E. de Vialar, 116 rue A. Charrial, 69003 Lyon, France
5
Accelerate Innovation Management SA, 1 rue de la Navigatino, 1201 Geneva, Switzerland
6
Centre Albert TRILLAT, 103 Grande rue de la Croix Rousse, 69004 Lyon, France
Introduction Various methods exist for measuring limb length and lateralisation after total hip arthroplasty (THA), most of which use standard antero-posterior (AP) pelvic radiographs [1–8]. Various anatomic landmarks and reference points have been described on those radiographs to obtain a number of measurements. Having the centres of rotation of both hips, the changes in limb length can be deduced from the distances between the centre of rotation and the tip of the lesser
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trochanter [1]. However the femoral lateralisation may alter this measurement. Limb length is more often measured by the distance between the tip of the lesser [9] or the greater trochanter [1, 10] and a horizontal line tangent to the inferior borders of both Bteardrops^. The results of this measurement can be influenced by the position of the limb being not strictly perpendicular to the pelvis horizontal reference during X-ray. Measurement of the femoral offset can be obtained by tracing a line parallel to the inter-teardrop line at the level of the lesser trochanter. On this line, the distance between the middle of the metaphysis and the vertical passing through the centre of rotation can be measured [10]. Similarly, the relative position of the femur in abduction or in adduction on the coronal plane may affect this measurement. The distance between the centre of rotation and the tip of the greater trochanter can also be measured, but the leg length may influence this measurement. While plain pelvic radiographs remains the standard to perform length and lateralisation’s femoral measurements, their accurate measure is subject to substantial error, if the individual pelvis and femur orientation with respect to the X-ray plate is not taken into consideration [11–13]. It is thus of special interest to control improved methods to accurately measure the post-operative femoral length and offset for a reasonable follow-up and the basis for any error analysis after THA. The aim of this study is to highlight the most accurate and reproducible method to measure limb length and femoral offset on AP pelvic X-ray radiographs post THA implantation by comparing variations in the results of different known measurement methods made in the same standard AP pelvic X-ray where femoral length, femoral offset and limb abduction or adduction were purposefully and precisely introduced.
Materials and methods AP pelvic X-ray radiographs post THA implantation remodelling (Fig. 1)
A standing pelvic AP radiograph post implantation of a right total hip prosthesis was digitised (Fig. 1a). An image editing software (Adobe Photoshop Element 2.0, San Jose, CA) was used to digitally segment the right femur and its femoral stem to the pelvis (Fig. 1b), such that they could be positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction with respect to the centre of rotation (Fig. 1c). Three different images of the same implant (identical length and offset) were therefore obtained (Fig. 2). This same implant was then digitally lengthened (Fig. 1d) giving three more images of a longer implant with the same offset and three positions, so-called pure lengthening
(Fig. 3). It was also lateralised with the same technique (Fig. 1e) giving three images of an implant of the same length with a larger offset and three positions, so-called pure lateralisation (Fig. 4). In total, we obtained nine images, which were randomly numbered (Table 1). The centres of rotation were traced in order to eliminate measurement biases. The contra-lateral hip was hidden. Femoral length measurement Three femoral length measurement methods were performed (Fig. 5): – – –
The distance between the centre of rotation and the tip of the lesser trochanter [1] (C-LT). The distance between the tip of the lesser trochanter and the horizontal passing through the centre of rotation [9] (HC-LT). The distance between the tip of the greater trochanter and the horizontal passing through the centre of rotation [1, 10] (HC-GT).
Femoral offset measurement Three femoral offset measurement methods were performed (Fig. 5): – –
–
The distance between the centre of rotation and the greater trochanter (C-GT), The distance, at the level of the lesser trochanter, between the middle of the metaphysis and the vertical (perpendicular to the teardrop line) passing through the centre of rotation [10] (VC-LT), The distance between the centre of rotation and the femoral anatomical axis [7, 14] (C-FA).
Twice four independent surgeons, who were kept unaware of the radiographs modifications and the study objectives, performed all measurements. Measurement analysis Two groups were assembled for the analysis of the results: –
–
One group of invariable length; where only the femoral position and implant lateralisation were changed (six different radiographs). In this group, variations in length measurements would correspond to biases induced by the measurement technique that was used. One group of invariable lateralisation; where only the femoral position and implant length were changed (six
International Orthopaedics (SICOT) Fig. 1 a Digitalisation of AP pelvic radiographs post implantation of a right THA. b image editing software used to digitally segment the right femur and its femoral stem to the pelvis. c femur and its femoral stem, digitally positioned orthogonal to the pelvis horizontal reference, with 10° of adduction, and with 10° of abduction. d femoral stem, digitally lengthened of 1 cm. e femoral stem, digitally lateralised of 1 cm
different radiographs). In this group, variations in femoral lateralisation measurements would correspond to biases induced by the measurement technique that was used.
excellent (0.81–1.00). Analysis of quantitative data was performed using non-parametric test (Mann–Whitney) with statistical significance accepted at p < 0.05.
Statistical analysis
Results
All analyses were performed with Microsoft Excel® 2008 and StatEl software (ad Science Society, Paris). We analysed Intra and inter-observer’s agreement using Kendall test. Kendall’s correlation coefficient was characterised as poor (0.00–0.20), fair (0.21–0.40), moderate (0.41–0.60), good (0.61–0.80) or
Group of invariable length
Fig. 2 Three different images of the same implant (identical length and offset) a right femur and its femoral stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
Intra and inter-observer discrepancies assessments on the length measurement showed that the measurement of distances C-LT, HC-LT and HC-GT are reproducible with
International Orthopaedics (SICOT) Fig. 3 Three different images of the same implant digitally lengthened (identical offset) a right femur and its femoral lengthen stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
excellent correlation (Kappa = 0.81, 0.87, 0.90 respectively). Maximum differences were less than 3 mm. Measurements results are summarised in Table 2. Within the invariable length group, modifying the femur position in adduction and abduction as well as offset augmentation significantly influenced the length measurements (Table 3): –
–
–
The C-LT value was significantly increased by an average of 3 mm per cm of lateralisation. However, this measurement was not affected by the femoral position in adduction or abduction (p = 0.09). The HC-LT value vas not influenced by lateralisation of the femur. However this measurement was significantly modified by femoral position (p < 0.001), with augmentation in adduction, and reduction in abduction, up to 4 mm with the use of a standard stem and up to 8 mm in case of 10 mm of lateralisation. The HC-GT value was the most sensitive to the variations of femoral position in adduction and abduction, with measurement deviations greater than 10 mm (p < 0.001). This value was also significantly influenced by femoral lateralisation.
Fig. 4 Three different images of the same implant digitally lateralised (identical length) a right femur and its femoral lateralised stem digitally positioned orthogonal to the pelvis horizontal reference, b with 10° of adduction, c with 10° of abduction, with respect to the centre of rotation
Group of invariable lateralisation Intra and inter-observer discrepancies assessments on the lateralisation measurement showed that the measurement of distances C-GT and VC-LT are reproducible with excellent correlation (Kappa = 0.88, 0.88 respectively) and maximum differences of 5 mm. Measurement of dist a n c e s C - FA w e r e f o u n d p o o r l y r e p r o d u c i b l e (Kappa = 0.20) (Table 2). Within the invariable lateralisation group, modifying the femoral position in adduction or in abduction as well as increasing the length significantly influenced some of the offset measurements (Table 4): – –
–
The C-GT value was reproducible and was not altered by length or femoral axis variations (p = 0.06). The VC-LT value was not influenced by length variations (p = 0.2), but an adducted femoral axis decreased significantly the offset measurement by 10 mm, and an abducted femoral axis increased it by 10 mm (p < 0.001). The C-FA value was not influenced by the various test configurations (p = 0.3) but still varies within 8 millimetres according to the intra and inter-observer discrepancies (Kappa = 0.20).
International Orthopaedics (SICOT) Table 1 Nine pelvic x-rays post THA implantation differing one to three variables
Table 2 Intra and inter-observer measurements variations with respect to the measurement method
Modification
Lateralisation
Length measurements
C-LT
HC-LT
HC-GT
Identical length and offset
Pure lengthenning
Pure lateralisation
Angle
Length
90°
0
Standard
Δ intra-observer
0.6 (0.6) 2
0.8 (0.8) 3
0.6 (0.6) 2
Adduction 10°
0
Standard
Δ inter-observer
2.1 (0,6) 3
2.2 (0.8) 3
1.9 (0.9) 3
Abduction 10° 90°
0 +10 mm
Standard Standard
Lateralisation measurements
C-GT
VC-LT
C-FA
Adduction 10°
+10 mm
Standard
Abduction 10° 90°
+10 mm 0
Standard Lateralised
Δ intra-observer Δ inter-observer
0.6 (0.6) 2 1.6 (0.6) 2
0.8 (0.7) 2 4 (0.6) 5
1.5 (1.5) 7 5.9 (1.3) 8
Adduction 10° Abduction 10°
0 0
Lateralised Lateralised
Discussion The present study did not attempt to assess the reliability of the landmarks used for the measurements, such as the identification of the tip of the lesser or greater trochanter. With the use of strictly identical radiographs, these landmarks are considered reliable, with inter-observer errors between 2 % and 7 %, and intra-observer between 1 % and 6 %, which corresponds approximately to 1 mm [1, 2]. Considering the measurements of the centre to the greater trochanter (C-GT) and centre to the lesser trochanter (C-LT), our results are consistent to previous studies with a excellent intra and inter-observer reliance. However, variations in the measurements reliability was founded poor in the C-FA measurement, with inter-observer errors of up to 8 mm. This probably reflects the poor accuracy in which the femoral anatomical axis can be identified. Moreover, a few tenths of degrees error on the axis measurement is bound to be amplified and will translate into
Average (Standard deviation) Maximal deviation Units = millimeters
millimetres on the measurements distant to the reference points. Among the available methods to measure leg length, the distance from the ischium line to the lesser trochanter was not tested. The ischium line is known to be highly influenced by changes in the pelvic rotation [15]. Conversely, the interteardrop line is not influenced by pelvic rotation. It is identified in several studies as the most reliable acetabular reference in the coronal plane [15, 16]. Clinically, how length variations are tolerated remains controversial. White and Dougall [2] did not find any correlation between leg length discrepancies at six months post op and HSS or SF 36 scores. For Konyves and Bannister [4], good leg length restoration is correlated to the OHS score. The preferred method found in the literature for leg length measurements after a hip prosthesis [2–4, 6, 8] was described and validated by Williamson and Reckling [9], who observed correlations close to those of lower limbs pangonograms. This technique measures the distance between the tip of the lesser trochanter and an inter-teardrop line (HC-LT). This measurement is reproducible and is only minimally altered by lateralisation. It is nevertheless modified by changes in the femur position in the coronal plan. This modification is further worsened by any increase in femoral lateralisation. Table 3 Average length measurements obtained with the various measurement methods, on invariable length images
Fig. 5 Femoral length and offset measurement methods. Purple = length measurement methods. Blue = offset measurement methods
Length measurements
C-LT
HC-LT
HC-GT
Standard stem— femur 90° Lateralised stem—femur 90° Standard stem—femur varus 10° Standard stem—femur valgus 10° Lateralised stem—femur varus 10°
57 (0.7) 60 (0.9) 58 (1.1) 57 (0.7) 59 (0.6)
56 (1.1) 55 (1.0) 57 (0.7) 52 (0.7) 57 (1.1)
−8 (0.4) −9 (0.6) 2 (1.2) −17 (0.6) 2 (0.9)
Lateralised stem—femur valgus 10° Max deviation
60 (0.5) 5
50 (0.6) 8
−19 (0.6) 24
Average (Standard deviation) Units = millimeters
International Orthopaedics (SICOT) Table 4 Average lateralisation measurements obtained with the various measurement methods, on invariable offset images Lateralisation measurements
C-GT
VC-LT
C-FA
Standard stem—femur 90°
54 (0.6)
44 (1.6)
45 (2.3)
Stem + 10 mm—Femur 90°
54 (0.7)
45 (1.7)
46 (1.4)
Standard stem—varus 10° Standard stem—valgus 10°
54 (0.7) 54 (0.6)
34 (2.0) 54 (1.4)
45 (2.6) 43 (1.9)
Stem + 10 mm—varus 10°
54 (0.4)
34 (1.7)
46 (2.2)
Stem + 10 mm—valgus 10° Max deviation
54 (0.6) 2
56 (1.7) 27
44 (2.4) 8
Average (Standard deviation) Units = millimeters
Another measurement method uses the distance between the tip of the greater trochanter and the horizontal passing through the centre of rotation [5, 10] (HC-GT). Just as the former, this measurement is only reliable on radiographs where there is no abduction or adduction of the femur. Any modification on the coronal plane alters the measurement by a delta (Δ) according to the formula: Δ = C-FA . sinα where the angle Balpha^ corresponds to the adduction angle. BAlpha^ angle is positive in adduction and negative in abduction (Fig. 6). The measurement is therefore considerably altered by the femoral angulation and offset, producing deviations up to 24 mm. Reducing these biases requires measurements on perfectly well taken radiographs. Nevertheless, pelvis deviations originating from the spine on a supine installation, or from leg length discrepancy on a standing radiograph, induce a lower limb deviation that is no longer perpendicular to the teardrop
Fig. 6 Measurement variations depending on femoral varus or valgus positioning of α angle to the pelvis horizontal reference. Length variation: ΔHC-GT = C-FA . sinα. Offset variation: Δ VC-LT = C-LT . sinα
line [11–13]. Such a deviation can produce errors when using pelvic-referenced measurements methods. The global offset is the sum of the femoral offset and the lateralisation of the centre of rotation. Charnley [17] was one of the first to describe how critical is the restoration of the femoral offset. Femoral offset’s influence on health-related quality of life after total hip replacement is no more controversial [7, 18]. Restoring femoral offset requires being perfectly planned pre-operatively because no reliable reference can be used to control it during the operation. The most widely used technique was published by Steinberg and Harris [7]. It corresponds to the distance between the femoral head centre and the femoral anatomical axis (C-FA). This measurement is not affected by the position of the femur. However, it remains poorly reproducible with kendall’s inter-observer correlation coefficient of 0.20 in our study, likely due to the lack of accuracy in identifying the femoral axis by standard AP x-rays. A computerised measurement involving automatic tracing of the femoral axis and its perpendiculars would probably improve its accuracy, but such techniques are still to be evaluated [8, 19–21]. Hausselle et al. [10] measured the offset with a technique to the VC-LT measurement in the present study. Since the reference is the pelvis, this measurement is reliable only on radiographs where the femoral anatomical axis is perpendicular to the pelvis horizontal. Placement of the femur in adduction or in abduction by an angle α will introduce an error of Δ according to the formula: Δ = C-LT . sinα (Fig. 6). As a result, this measurement is altered by the femoral position and also by the variation in the femoral length. In the present study, deviations for identical lateralisation can reach 27 mm. The most reliable results are obtained with the measurement of CGT in the evaluation of the femoral offset, with maximum deviations of only 2 mm. This simple measurement is not influenced by length or femoral axis variations. However, limitations in the reliability of the measurements from bi-dimensional data still persist [13]. If the radiological evaluation is performed on planar projections, any variation of the femoral stem anteversion alters the offset measurement proportionally to the cosine of the anteversion angle. A 20° anteversion underestimates femoral offset by 15 % [10]. For this reason, it is critical to measure on an X-ray where the femoral anteversion is compensated by proper limb rotation. When the offset of the implanted stem is known, it is possible to check if the prosthetic neck has been radiographed in a true frontal view, and if not, its anteversion α can be worked out with the formula: cosα = Xray measurement/stem offset. The goal is still to compare the operated limb offset and length to the contralateral side. Bony landmarks to visualise the trochanters are critical to screen out radiographs where both femurs would not appear in the same rotation. In those cases, comparative measurements cannot be achieved. With its three-dimensional abilities, tomodensitometry, associated
International Orthopaedics (SICOT)
to the relevant software, is the reference tool to measure lengths, lateralisations and rotations of implants [22, 23]. This study identifies the limitations of some measurement techniques with regards to their poor reliability and reproducibility and demonstrates that regardless of the method, some degree of inaccuracy persists. In conclusion, femoral length may not be possibly measured with accuracy greater than 5 mm on standard AP pelvic X-rays. It is best obtained from the distance between the centre of rotation and the tip of the lesser trochanter (C-LT), provided that femoral offset has not changed more than 10 mm. Femoral offset can be measured with an excellent accuracy of approximately 2 mm from the distance between the centre of rotation and the greater trochanter (C-GT), provided that the x-rays are acquired with a true frontal view of the femoral neck. Compliance with ethical standards Conflict of interest and funding No funds were received in support of this study. No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.
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References 1.
2. 3.
4.
5.
6.
7.
Eggli S, Pisan M, Muller ME (1998) The value of preoperative planning for total hip arthroplasty. J Bone Joint Surg (Br) 80(3): 382–390 White TO, Dougall TW (2002) Arthroplasty of the hip. Leg length is not important. J Bone Joint Surg (Br) 84(3):335–338 Della Valle AG, Padgett DE, Salvati EA (2005) Preoperative planning for primary total hip arthroplasty. J Am Acad Orthop Surg 13(7):455–462 Konyves A, Bannister GC (2005) The importance of leg length discrepancy after total hip arthroplasty. J Bone Joint Surg (Br) 87(2):155–157 Debarge R, Lustig S, Neyret P, Ait Si Selmi T (2008) Confrontation of the radiographic preoperative planning with the postoperative data for uncemented total hip arthroplasty. Rev Chir Orthop Reparatrice Appar Mot 94(4):368–375 Unnanuntana A, Wagner D, Goodman SB (2009) The accuracy of preoperative templating in cementless total hip arthroplasty. J Arthroplasty 24(2):180–186 Steinberg B, Harris, W (1992) The offset problem in total hip arthroplasty. Contemp Orthop 24:556
17. 18.
19.
20.
21.
22.
23.
Shaarani SR, McHugh G, Collins DA (2013) Accuracy of digital preoperative templating in 100 consecutive uncemented total hip arthroplasties: a single surgeon series. J Arthroplasty 28(2):331– 337 Williamson JA, Reckling FW (1978) Limb length discrepancy and related problems following total hip joint replacement. Clin Orthop Relat Res 134:135–138 Hausselle J, Moreau PE, Wessely L, de Thomasson E, Assi A, Parratte S, Essig J, Skalli W (2012) Intra- and extra-articular planes of reference for use in total hip arthroplasty: a preliminary study. Int Orthop 36(8):1567–1573 Craiovan B, Renkawitz T, Weber M, Grifka J, Nolte L, Zheng G (2014) Is the acetabular cup orientation after total hip arthroplasty on a two dimension or three dimension model accurate? Int Orthop 38(10):2009–2015 Kim YH, Cho KH, Park YG (2015) Is the acetabular cup orientation after total hip arthroplasty on a two-dimensional or three-dimensional model accurate? Int Orthop 39(4):819– 820 Tsai TY, Dimitriou D, Li G, Kwon YM (2014) Does total hip arthroplasty restore native hip anatomy? three-dimensional reconstruction analysis. Int Orthop 38(8):1577–1583 Schmidutz F, Beirer M, Weber P, Mazoochian F, Fottner A, Jansson V (2012) Biomechanical reconstruction of the hip: comparison between modular short-stem hip arthroplasty and conventional total hip arthroplasty. Int Orthop 36(7):1341–1347 Goodman SB, Adler SJ, Fyhrie DP, Schurman DJ (1988) The acetabular teardrop and its relevance to acetabular migration. Clin Orthop Relat Res 236:199–204 Meermans G, Malik A, Witt J, Haddad F (2011) Preoperative radiographic assessment of limb-length discrepancy in total hip arthroplasty. Clin Orthop Relat Res 469(6):1677–1682 Charnley J (1979) Low friction arthroplasty of the hip: theory and practice. Springer, New York Liebs TR, Nasser L, Herzberg W, Ruther W, Hassenpflug J (2014) The influence of femoral offset on health-related quality of life after total hip replacement. Bone Joint J 96-B(1):36–42 Gamble P, de Beer J, Petruccelli D, Winemaker M (2010) The accuracy of digital templating in uncemented total hip arthroplasty. J Arthroplasty 25(4):529–532 Kumar PG, Kirmani SJ, Humberg H, Kavarthapu V, Li P (2009) Reproducibility and accuracy of templating uncemented THA with digital radiographic and digital TraumaCad templating software. Orthopedics 32(11):815 The B, Verdonschot N, van Horn JR, van Ooijen PM, Diercks RL (2007) Digital versus analogue preoperative planning of total hip arthroplasties: a randomized clinical trial of 210 total hip arthroplasties. J Arthroplasty 22(6):866–870 Sariali E, Mouttet A, Pasquier G, Durante E, Catone Y (2009) Accuracy of reconstruction of the hip using computerised threedimensional pre-operative planning and a cementless modular neck. J Bone Joint Surg (Br) 91(3):333–340 Pasquier G, Ducharne G, Ali ES, Giraud F, Mouttet A, Durante E (2010) Total hip arthroplasty offset measurement: is C T scan the most accurate option? Orthop Traumatol Surg Res 96(4):367–375
