Arch Orthop Trauma Surg (2014) 134:91–97 DOI 10.1007/s00402-013-1896-1
KNEE ARTHROPLASTY
Alteration of the patellar height following total knee arthroplasty A. Jawhar · S. Sohoni · V. Shah · H. P. Scharf
Received: 20 May 2013 / Published online: 28 November 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Introduction Patellofemoral related complications after total knee arthroplasty (TKA) remain clinically relevant. The hypothesis of the present study was that the patellar height changes more than 10 % of its preoperative height after TKA. Possible influences of age, gender, side and navigation system on patellar height were evaluated separately in subgroups. Materials and methods A total of 107 knees were enrolled after primary TKA. The patellar height was determined for each patient preoperatively, 1 week and 1 year postoperatively on routinely performed standing lateral view radiographs at 30° knee flexion. Insall–Salvati index (ISI), modified Insall–Salvati index (MIS) as well as Miura–Kawamura index (MKI) were determined for the whole cohort. Results One week after TKA the ISI, MIS and MKI changed by more than 10 % in 24 (22 %), 33 (30 %) and 54 (50 %) cases, respectively. Moreover, the 1 year follow-up revealed a decrease or increase of ISI in 30 (28 %), MIS in 47 (44 %) and MK in 65 (61 %) knees. The frequency of patella alta, norma and baja preoperatively as well as 1 week and 1 year postoperatively were not significantly different. Significant differences of patellar height changes were not noted between the defined subgroups. A. Jawhar (*) · H. P. Scharf Orthopaedic and Trauma Surgery Center, University Medical Center Mannheim of University Heidelberg, Theodor‑Kutzer‑Ufer 1–3, 68167 Mannheim, Germany e-mail: [email protected] S. Sohoni Kaushalya Medical Foundation Trust Hospital, Thane, Maharashtra, India V. Shah Navrang Hospital, Vadodara, Gujarat, India
Conclusions The present study demonstrates that TKA leads, at 1 week and 1 year follow-up, to patellar height alteration more than 10 % in a significant number of knee joints. However, with the use of ISI and MIS the changes of patellar height did not exceed the defined thresholds to be classified as patella alta or baja. Keywords Patellar height · Patella alta · Patella baja · Total knee arthroplasty
Introduction Total knee arthroplasty (TKA) is a well established procedure with satisfactory clinical results for the treatment of symptomatic osteoarthritis. Despite the efforts to improve surgical techniques and implant designs, there are still a significant number of patients suffering from anterior knee pain, which might be related to postoperative changes of patellar height with consecutive influence on the patellofemoral compartment [1–7]. The changes of patellar height might occur immediately after TKA due to implant positioning or develop during the postoperative time period. Currently discussed risk factors of patellar height changes postoperatively are scare tissue formation [8], removal of Hoffa’s fat pad [9], surgical approach [10] and previous operations such as ACLreconstruction [11] or high tibial osteotomy [12]. Lower functional scores [13] and stiff knee [14] have been reported after change of patellar height. Weale et al. [15] defined a patellar tendon shortening by 10 % and more as patella infera, they found significant correlation between patellar tendon shortening and loss of knee flexion. They described a loss of flexion of 1° for every millimeter of patellar tendon shortening.
13
92
To compare our results to previously published works a patellar height change of more than 10 % was defined as a cutoff level [1, 10, 15, 16]. Unclear is the frequency of patellar height changes immediately after TKA and whether the changes increase during the postoperative time. The hypothesis of the present study was that the patellar height changes more than ±10 % in a significant number of cases 1 week and 1 year after TKA. The number of knees with a change of the patellar height more than ±10 % increase after 1 year as compared to 1 week postoperative follow-up.
Materials and methods In this retrospective cohort study 107 patients with symptomatic osteoarthritis underwent primary TKA. Five cases were excluded in advance due to inadequate radiographs, history of ACL reconstruction or high tibial osteotomy. The demographics and implanted prosthesis designs are visualized (Table 1). All procedures were performed under tourniquet with medial parapatellar approach and gap balancing technique. Hoffa’s fat pad has been partly removed in all cases and the patella was retracted or reduced but not everted depending on the operation steps. Three degree tibial slope was set for all cases. The final leg alignment, stability, range of motion, flexion and extension gap heights were checked with trial implants before cementing of femoral and tibial components. The patellar height was measured for each patient preoperatively, 1 week and 1 year postoperatively on routinely performed standing lateral view radiographs at approximately 30° knee flexion. A goniometer was applied to determine the flexion degree and standards of performing lateral view knee X-rays [17] were considered. A digital image viewer (Syngo fastview program)1 was applied to optimize the soft tissue contrast before carrying out the measurements. The knees were also categorized into groups according to age (>65 vs. 1.2 patella alta and 2 defines patella alta [19]. In addition a technique, which has been first described by MK was used to measure the patellar height directly. This index establishes a relationship between femur and patella as it includes the distance between femoral joint line and the inferior aspect of patellar articular surface divided by the length of the patellar articular surface. Normal values are not available for this index. Miura-Kawamura et al. [20] considered a change more than 10 % from pre to 2 weeks postoperative to be significant. The reliability of the three utilized methods has been considered, as two observers have conducted their preoperative and 1 week postoperative measurements for the first ten cases twice independently. In order to reduce error due to difficult identification of the landmarks all three radiographs for each patient were reviewed by two authors simultaneously. Since the radiographs were performed routinely, an ethical approval was not necessary. Statistical analysis For each measurement mean, median, standard deviation, distribution and range were calculated. Statistical analysis of the patellar height changes was made using student’s t test, since the values were normally distributed (Excel Version 2007: Normvert). Statistical significant was set at a p-value of 10 % change of patellar height (Table 4). The preoperative and postoperative values of ISI and MIS classified as patella alta, normal or infera are represented (Table 3). Miura–Kawamura index (MKI) The average preoperative, 1 week postoperative and 1 year postoperative MK indices were calculated to be 0.91 (range 0.61–1.14), 0.87 (range 0.5–1.3) and 0.87 (range 0.28–1.11). A statistically significant difference was found for the patellar height changes in the time periods preoperative–1 week postoperative (p 0.03) as well as preoperative–1 year postoperative (p 0.01). Between 1 week and 1 year follow-up there was not a significant difference noted (p 0.82). TKA surgery resulted in a change of the patellar height more than 10 % according to MKI in 54 (50 %) knees after 1 week and 65 (60 %) knees after 1 year (Table 4).
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Arch Orthop Trauma Surg (2014) 134:91–97
Table 3 Classification of patella height with two methods Preoperative
1 week postoperative
1 year postoperative
Insall–Salvati index 9
7
8
96
99
99
2 Patella baja (n) Modified Insall–Salvati 2 Patella alta (n)
1
0
0
3
105
107
104
Patella alta (n) Patella norma (n)
Patella norma (n) Patella baja (n)
–
–
–
Table 4 Distribution of the Patellar height changes 1 week and 1 year postoperatively Patellar height changes
ISI 1 week 1 year MIS 1 week 1 year MKI 1 week 1 year
≤10 %
>10 %
83 77
24 30
74 60
33 47
53
54
42
65
All analyzed factors such as gender, age, side and operation techniques failed to show significant differences of relative patellar height changes after 1 week and 1 year follow-up (Table 5).
Discussion Several theories such as alteration of tibial slope, shift of joint line, cast immobilization, scare tissue or new bone formation, intratendinous fibrosis, tendon adhesion at infrapatellar bursa or proximal part of tuberositas tibia, trauma and ischemia have been introduced to clarify the etiology of patellar height reduction [15]. Even though the cause of the patellar tendon shrinking or elongation is not completely understood, the clinical impacts are well documented. Patients with patella infera reported to suffer from anterior knee pain and reduced range of joint movement [2–4, 13, 15]. On the other hand patella alta is associated with patellar dislocation [23]. The main findings of this study is that TKA leads at 1 week and 1 year follow-up to patellar height shift more than 10 % in a significant number of patients as measured
13
by ISI, MIS and MKI. To our knowledge this study is the first using three different patellar height measuring methods to evaluate knees after TKA. The number of knees with change of the patellar height more than ±10 % increase after 1 year as compared to 1 week postoperative follow-up. For the purpose of patellar height determination many different radiographic measuring techniques have been used and reviewed [24]. The most popular one is the ISI. The insertion point of the patellar tendon is sometimes difficult to assess, however, modern radiographs provide a satisfactory soft tissue contrast to allow a more precise identification of the landmarks. One other drawback is the anatomical variability of patellar shape, which is not taken into account by ISI. The MIS was applied concomitantly as recommended [19] to improve the sensitivity for identification of patellar height pathology. MK was considered as an additional measuring method, which contains the disadvantage of joint line dependency. Seil et al. [25] evaluated five patellar height ratios including ISI and MIS as measured by two examiners. Their results revealed a low interobserver variability with high correlation coefficients for ISI 0.86 and MIS 0.82. BP or CD methods have been reported to be highly reliable [25, 26], however, they are not suitable for our study, since their tibial landmarks get removed during TKA and replaced by an radiologically invisible polyethylene insert. The BP and CD methods also contain the disadvantage of joint line dependency due to its tibial plateau reference and can exhibit a low value with true patella baja and pseudo-patella baja [10]. The overall preoperative value for the patellar height determined with ISI was 1.07 ± 0.11. The normal range for ISI have been reported from the original paper to be 1.02 ± 0.13, which was confirmed by the anatomical study of Schlenzka et al. [27] and other authors [2, 28–30]. The results of Grelsamer et al. for MIS averaged 1.75 range (1.2–2.6) corresponded well to our findings 1.78 range (1.42–2.04). Our results according to the MK ratio were 0.91 range (0.61–1.14), similar values have been reported for the height of the patella measured by this joint line depending method [20, 31]. Our study demonstrates clearly that after TKA there was no statistical significant alteration in the patellar height occurred 1 week and 1 year after surgery when established methods such as ISI and MIS were applied and their defined limits for patella normal, alta and baja were considered [18, 19]. However, if defining changes more than 10 % to be clinically relevant, ISI decrease or increase was detectable in 24 (22 %) knees at 1 week and in 30 (33 %) knees at 1 year postoperative assessment. According to MIS, our results reveal a decrease or increase by more than 10 % of the values in 33 (31 %) and 47 (44 %) subjects at 1 week and 1 year follow-up, respectively. If considering the MKI as
95
Arch Orthop Trauma Surg (2014) 134:91–97
Table 5 Subgroup (gender, age, side, navigation) specific evaluation of the relative patellar height differences (%) (1 week postop–preop and 1 year postop–preop) with three methods ISI
Gender Female Male p-value Age 65 p-value Side Left Right
MKI
1 week post–preop Mean ± std (%)
1 year post–preop mean ± std (%)
1 week post–preop mean ± std (%)
1 year post–preop mean ± std (%)
1 week post–preop mean ± std (%)
1 year post–preop mean ± std (%)
6 ± 5 6 ± 5 0.72
6 ± 6 8 ± 7 0.19
6 ± 5 9 ± 7 0.07
8 ± 6 10 ± 6 0.14
14 ± 11 11 ± 8 0.24
14 ± 11 15 ± 11 0.72
6 ± 4 6 ± 5 0.9
6 ± 6 8 ± 7 0.27
8 ± 7 7 ± 5 0.15
9 ± 7 9 ± 6 0.73
15 ± 12 11 ± 9 0.1
16 ± 13 14 ± 10 0.25
6 ± 5 6 ± 4 0.35
7 ± 7 7 ± 6 0.77
8 ± 7 7 ± 5 0.44
9 ± 6 10 ± 6 0.73
12 ± 11 13 ± 9 0.6
12 ± 9 13 ± 12 0.35
7 ± 6 8 ± 7
7 ± 6 7 ± 6
9 ± 6 10 ± 7
11 ± 9 15 ± 11
14 ± 11 15 ± 10
0.43
0.96
0.26
0.14
0.53
P-value Navigation No 6 ± 5 Yes 6 ± 5 p-value
MIS
0.9
Mean and standard deviation (std) of relative patellar height differences (%) are calculated for the defined time periods
a direct technique using femoral and patellar landmarks, a statistically significant MKI decrease could be noted within time periods preop–1 week and preop–1 year, whereas for 1 week–1 year assessment no significant changes were found. Besides the patellar tendon shrinking, the elevation of the joint line leads to decrease in MKI. This factor may provide an explanation for our findings as a joint line shift usually occurs immediately after TKA and is not expected to change after 1 year. Gatha et al. [6] followed 135 patients undergoing 154 primary TKAs and found no significant difference between preoperative and postoperative patellar height as measured by ISI or BP ratio. Koshino et al. [1] included a total of 94 TKAs and found at 1 month no changes of the patellar height. However, they reported a statistically significant decrease of the patellar height determined with ISI within the time periods 6 months, 1, 2 and more than 3 years. A decrease in the ISI by more than 10 % was detected in 61 cases (64.9 %), they reported, however, that patella infera after TKA did not affect range of movement or quadriceps muscle power [1]. Weale et al. [15] conducted a randomized controlled study investigating the length of the patellar tendon after unicompartmental and TKA in 84 patients. Patella infera was defined as tendon shortening by 10 % and more. After TKA 34 % of the knees developed patella infera at 5 years follow-up. The incidence of patella infera was higher in knees when a lateral release was performed [15]. Sharma et al. [16] reported no clinically adverse effect if
the reduction in the ISI occurred without patella baja. Furthermore in a retrospective review Meneghini et al. [13] look at the effect of ISI on outcome after TKA. An ISI increase or decrease was observed in 30.2 or 50 %, respectively. After excluding the cases with preoperative patella infera, 92 knees (9.8 %) developed postoperative patella infera (ratio
KNEE ARTHROPLASTY
Alteration of the patellar height following total knee arthroplasty A. Jawhar · S. Sohoni · V. Shah · H. P. Scharf
Received: 20 May 2013 / Published online: 28 November 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Introduction Patellofemoral related complications after total knee arthroplasty (TKA) remain clinically relevant. The hypothesis of the present study was that the patellar height changes more than 10 % of its preoperative height after TKA. Possible influences of age, gender, side and navigation system on patellar height were evaluated separately in subgroups. Materials and methods A total of 107 knees were enrolled after primary TKA. The patellar height was determined for each patient preoperatively, 1 week and 1 year postoperatively on routinely performed standing lateral view radiographs at 30° knee flexion. Insall–Salvati index (ISI), modified Insall–Salvati index (MIS) as well as Miura–Kawamura index (MKI) were determined for the whole cohort. Results One week after TKA the ISI, MIS and MKI changed by more than 10 % in 24 (22 %), 33 (30 %) and 54 (50 %) cases, respectively. Moreover, the 1 year follow-up revealed a decrease or increase of ISI in 30 (28 %), MIS in 47 (44 %) and MK in 65 (61 %) knees. The frequency of patella alta, norma and baja preoperatively as well as 1 week and 1 year postoperatively were not significantly different. Significant differences of patellar height changes were not noted between the defined subgroups. A. Jawhar (*) · H. P. Scharf Orthopaedic and Trauma Surgery Center, University Medical Center Mannheim of University Heidelberg, Theodor‑Kutzer‑Ufer 1–3, 68167 Mannheim, Germany e-mail: [email protected] S. Sohoni Kaushalya Medical Foundation Trust Hospital, Thane, Maharashtra, India V. Shah Navrang Hospital, Vadodara, Gujarat, India
Conclusions The present study demonstrates that TKA leads, at 1 week and 1 year follow-up, to patellar height alteration more than 10 % in a significant number of knee joints. However, with the use of ISI and MIS the changes of patellar height did not exceed the defined thresholds to be classified as patella alta or baja. Keywords Patellar height · Patella alta · Patella baja · Total knee arthroplasty
Introduction Total knee arthroplasty (TKA) is a well established procedure with satisfactory clinical results for the treatment of symptomatic osteoarthritis. Despite the efforts to improve surgical techniques and implant designs, there are still a significant number of patients suffering from anterior knee pain, which might be related to postoperative changes of patellar height with consecutive influence on the patellofemoral compartment [1–7]. The changes of patellar height might occur immediately after TKA due to implant positioning or develop during the postoperative time period. Currently discussed risk factors of patellar height changes postoperatively are scare tissue formation [8], removal of Hoffa’s fat pad [9], surgical approach [10] and previous operations such as ACLreconstruction [11] or high tibial osteotomy [12]. Lower functional scores [13] and stiff knee [14] have been reported after change of patellar height. Weale et al. [15] defined a patellar tendon shortening by 10 % and more as patella infera, they found significant correlation between patellar tendon shortening and loss of knee flexion. They described a loss of flexion of 1° for every millimeter of patellar tendon shortening.
13
92
To compare our results to previously published works a patellar height change of more than 10 % was defined as a cutoff level [1, 10, 15, 16]. Unclear is the frequency of patellar height changes immediately after TKA and whether the changes increase during the postoperative time. The hypothesis of the present study was that the patellar height changes more than ±10 % in a significant number of cases 1 week and 1 year after TKA. The number of knees with a change of the patellar height more than ±10 % increase after 1 year as compared to 1 week postoperative follow-up.
Materials and methods In this retrospective cohort study 107 patients with symptomatic osteoarthritis underwent primary TKA. Five cases were excluded in advance due to inadequate radiographs, history of ACL reconstruction or high tibial osteotomy. The demographics and implanted prosthesis designs are visualized (Table 1). All procedures were performed under tourniquet with medial parapatellar approach and gap balancing technique. Hoffa’s fat pad has been partly removed in all cases and the patella was retracted or reduced but not everted depending on the operation steps. Three degree tibial slope was set for all cases. The final leg alignment, stability, range of motion, flexion and extension gap heights were checked with trial implants before cementing of femoral and tibial components. The patellar height was measured for each patient preoperatively, 1 week and 1 year postoperatively on routinely performed standing lateral view radiographs at approximately 30° knee flexion. A goniometer was applied to determine the flexion degree and standards of performing lateral view knee X-rays [17] were considered. A digital image viewer (Syngo fastview program)1 was applied to optimize the soft tissue contrast before carrying out the measurements. The knees were also categorized into groups according to age (>65 vs. 1.2 patella alta and 2 defines patella alta [19]. In addition a technique, which has been first described by MK was used to measure the patellar height directly. This index establishes a relationship between femur and patella as it includes the distance between femoral joint line and the inferior aspect of patellar articular surface divided by the length of the patellar articular surface. Normal values are not available for this index. Miura-Kawamura et al. [20] considered a change more than 10 % from pre to 2 weeks postoperative to be significant. The reliability of the three utilized methods has been considered, as two observers have conducted their preoperative and 1 week postoperative measurements for the first ten cases twice independently. In order to reduce error due to difficult identification of the landmarks all three radiographs for each patient were reviewed by two authors simultaneously. Since the radiographs were performed routinely, an ethical approval was not necessary. Statistical analysis For each measurement mean, median, standard deviation, distribution and range were calculated. Statistical analysis of the patellar height changes was made using student’s t test, since the values were normally distributed (Excel Version 2007: Normvert). Statistical significant was set at a p-value of 10 % change of patellar height (Table 4). The preoperative and postoperative values of ISI and MIS classified as patella alta, normal or infera are represented (Table 3). Miura–Kawamura index (MKI) The average preoperative, 1 week postoperative and 1 year postoperative MK indices were calculated to be 0.91 (range 0.61–1.14), 0.87 (range 0.5–1.3) and 0.87 (range 0.28–1.11). A statistically significant difference was found for the patellar height changes in the time periods preoperative–1 week postoperative (p 0.03) as well as preoperative–1 year postoperative (p 0.01). Between 1 week and 1 year follow-up there was not a significant difference noted (p 0.82). TKA surgery resulted in a change of the patellar height more than 10 % according to MKI in 54 (50 %) knees after 1 week and 65 (60 %) knees after 1 year (Table 4).
13
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Arch Orthop Trauma Surg (2014) 134:91–97
Table 3 Classification of patella height with two methods Preoperative
1 week postoperative
1 year postoperative
Insall–Salvati index 9
7
8
96
99
99
2 Patella baja (n) Modified Insall–Salvati 2 Patella alta (n)
1
0
0
3
105
107
104
Patella alta (n) Patella norma (n)
Patella norma (n) Patella baja (n)
–
–
–
Table 4 Distribution of the Patellar height changes 1 week and 1 year postoperatively Patellar height changes
ISI 1 week 1 year MIS 1 week 1 year MKI 1 week 1 year
≤10 %
>10 %
83 77
24 30
74 60
33 47
53
54
42
65
All analyzed factors such as gender, age, side and operation techniques failed to show significant differences of relative patellar height changes after 1 week and 1 year follow-up (Table 5).
Discussion Several theories such as alteration of tibial slope, shift of joint line, cast immobilization, scare tissue or new bone formation, intratendinous fibrosis, tendon adhesion at infrapatellar bursa or proximal part of tuberositas tibia, trauma and ischemia have been introduced to clarify the etiology of patellar height reduction [15]. Even though the cause of the patellar tendon shrinking or elongation is not completely understood, the clinical impacts are well documented. Patients with patella infera reported to suffer from anterior knee pain and reduced range of joint movement [2–4, 13, 15]. On the other hand patella alta is associated with patellar dislocation [23]. The main findings of this study is that TKA leads at 1 week and 1 year follow-up to patellar height shift more than 10 % in a significant number of patients as measured
13
by ISI, MIS and MKI. To our knowledge this study is the first using three different patellar height measuring methods to evaluate knees after TKA. The number of knees with change of the patellar height more than ±10 % increase after 1 year as compared to 1 week postoperative follow-up. For the purpose of patellar height determination many different radiographic measuring techniques have been used and reviewed [24]. The most popular one is the ISI. The insertion point of the patellar tendon is sometimes difficult to assess, however, modern radiographs provide a satisfactory soft tissue contrast to allow a more precise identification of the landmarks. One other drawback is the anatomical variability of patellar shape, which is not taken into account by ISI. The MIS was applied concomitantly as recommended [19] to improve the sensitivity for identification of patellar height pathology. MK was considered as an additional measuring method, which contains the disadvantage of joint line dependency. Seil et al. [25] evaluated five patellar height ratios including ISI and MIS as measured by two examiners. Their results revealed a low interobserver variability with high correlation coefficients for ISI 0.86 and MIS 0.82. BP or CD methods have been reported to be highly reliable [25, 26], however, they are not suitable for our study, since their tibial landmarks get removed during TKA and replaced by an radiologically invisible polyethylene insert. The BP and CD methods also contain the disadvantage of joint line dependency due to its tibial plateau reference and can exhibit a low value with true patella baja and pseudo-patella baja [10]. The overall preoperative value for the patellar height determined with ISI was 1.07 ± 0.11. The normal range for ISI have been reported from the original paper to be 1.02 ± 0.13, which was confirmed by the anatomical study of Schlenzka et al. [27] and other authors [2, 28–30]. The results of Grelsamer et al. for MIS averaged 1.75 range (1.2–2.6) corresponded well to our findings 1.78 range (1.42–2.04). Our results according to the MK ratio were 0.91 range (0.61–1.14), similar values have been reported for the height of the patella measured by this joint line depending method [20, 31]. Our study demonstrates clearly that after TKA there was no statistical significant alteration in the patellar height occurred 1 week and 1 year after surgery when established methods such as ISI and MIS were applied and their defined limits for patella normal, alta and baja were considered [18, 19]. However, if defining changes more than 10 % to be clinically relevant, ISI decrease or increase was detectable in 24 (22 %) knees at 1 week and in 30 (33 %) knees at 1 year postoperative assessment. According to MIS, our results reveal a decrease or increase by more than 10 % of the values in 33 (31 %) and 47 (44 %) subjects at 1 week and 1 year follow-up, respectively. If considering the MKI as
95
Arch Orthop Trauma Surg (2014) 134:91–97
Table 5 Subgroup (gender, age, side, navigation) specific evaluation of the relative patellar height differences (%) (1 week postop–preop and 1 year postop–preop) with three methods ISI
Gender Female Male p-value Age 65 p-value Side Left Right
MKI
1 week post–preop Mean ± std (%)
1 year post–preop mean ± std (%)
1 week post–preop mean ± std (%)
1 year post–preop mean ± std (%)
1 week post–preop mean ± std (%)
1 year post–preop mean ± std (%)
6 ± 5 6 ± 5 0.72
6 ± 6 8 ± 7 0.19
6 ± 5 9 ± 7 0.07
8 ± 6 10 ± 6 0.14
14 ± 11 11 ± 8 0.24
14 ± 11 15 ± 11 0.72
6 ± 4 6 ± 5 0.9
6 ± 6 8 ± 7 0.27
8 ± 7 7 ± 5 0.15
9 ± 7 9 ± 6 0.73
15 ± 12 11 ± 9 0.1
16 ± 13 14 ± 10 0.25
6 ± 5 6 ± 4 0.35
7 ± 7 7 ± 6 0.77
8 ± 7 7 ± 5 0.44
9 ± 6 10 ± 6 0.73
12 ± 11 13 ± 9 0.6
12 ± 9 13 ± 12 0.35
7 ± 6 8 ± 7
7 ± 6 7 ± 6
9 ± 6 10 ± 7
11 ± 9 15 ± 11
14 ± 11 15 ± 10
0.43
0.96
0.26
0.14
0.53
P-value Navigation No 6 ± 5 Yes 6 ± 5 p-value
MIS
0.9
Mean and standard deviation (std) of relative patellar height differences (%) are calculated for the defined time periods
a direct technique using femoral and patellar landmarks, a statistically significant MKI decrease could be noted within time periods preop–1 week and preop–1 year, whereas for 1 week–1 year assessment no significant changes were found. Besides the patellar tendon shrinking, the elevation of the joint line leads to decrease in MKI. This factor may provide an explanation for our findings as a joint line shift usually occurs immediately after TKA and is not expected to change after 1 year. Gatha et al. [6] followed 135 patients undergoing 154 primary TKAs and found no significant difference between preoperative and postoperative patellar height as measured by ISI or BP ratio. Koshino et al. [1] included a total of 94 TKAs and found at 1 month no changes of the patellar height. However, they reported a statistically significant decrease of the patellar height determined with ISI within the time periods 6 months, 1, 2 and more than 3 years. A decrease in the ISI by more than 10 % was detected in 61 cases (64.9 %), they reported, however, that patella infera after TKA did not affect range of movement or quadriceps muscle power [1]. Weale et al. [15] conducted a randomized controlled study investigating the length of the patellar tendon after unicompartmental and TKA in 84 patients. Patella infera was defined as tendon shortening by 10 % and more. After TKA 34 % of the knees developed patella infera at 5 years follow-up. The incidence of patella infera was higher in knees when a lateral release was performed [15]. Sharma et al. [16] reported no clinically adverse effect if
the reduction in the ISI occurred without patella baja. Furthermore in a retrospective review Meneghini et al. [13] look at the effect of ISI on outcome after TKA. An ISI increase or decrease was observed in 30.2 or 50 %, respectively. After excluding the cases with preoperative patella infera, 92 knees (9.8 %) developed postoperative patella infera (ratio
