EDITORIAL
Can we identify and predict failure of knee implants from routine bone density measurements? T. J. Wilton From Royal Derby Hospital, Derby, United Kingdom
T. J. Wilton, FRCS, Consultant Orthopaedic Surgeon Royal Derby Hospital, Uttoxeter Road, Derby, UK. Correspondence should be sent to Mr T. J. Wilton; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B11. 34982 $2.00 Bone Joint J 2014;96-B:1429–30.
In this edition readers will find an interesting paper concerning the quality and density of bone surrounding the tibial implant of total knee replacements.1 This paper is interesting for a number of reasons. Firstly, the authors have been able to identify bone density changes and follow them in a quantitative manner from the routine post-operative radiographs obtained in the orthopaedic clinic. Secondly, they have identified density changes which occurred differentially in their cases which developed failure of the implant. Finally, they were able to identify different changes in the tibial density depending upon the type of failure which subsequently occurred in their patients’ knees. The use of routinely available radiographs clearly offers some substantial advantages over the use of DEXA scanning for this purpose. Most health services would find the use of routine and multiple DEXA scans prohibitively expensive and cumbersome in this context, even as a research tool quite apart from their use as a routine monitoring system. The system these authors used for identifying the density changes used digitised radiographs and has been previously reported by them.2 The system is modified in this paper to compare the cases whose knees failed with case-matched controls and then the whole series of radiographs is followed to identify changes in density over time. Finally parts of the implant are also used to give an ‘absolute’ density standardisation. The findings suggest that in knees that do not fail, there is a gradual decline in bone density beneath the tibial component both medially and laterally for several years after operation. This is expected as it simply reflects the stress protection that is known to occur in bone around knee implants and which has been described in detail by Walker many years ago.3,4 Those knees that failed due to instability appeared to have still lower density but this was similar on medial and lateral sides of the
VOL. 96-B, No. 11, NOVEMBER 2014
knee and this was only statistically significant shortly after the surgery. The knees failing by medial collapse and subsidence of the tibia, and those which failed with progressive radiolucent lines, had increased in the relative density on the medial side of the tibia even by two months post-operatively, and this relative increase compared with the control knees continued for several years after surgery. The authors consider that these findings indicate that there is overloading of the medial side of the knee in those knees whose implants collapse in varus and those which develop progressive radiolucency and therefore the medial tibia undergoes increase in density according to Wolff’s law. In those which loosen, this increased density proves insufficient to prevent the overload of the bone so that collapse and loosening occur anyway. This is a plausible explanation, but perhaps not the only possible explanation. We would like to know whether bone density changes are responsible for the loosening and collapse of tibial components of TKR, though it is important to realise that bone density and bone strength are not synonymous. This is true in part because of the complex relationship between bone size and patient size, but has also been demonstrated by the work of Ralis5 on Bone Quality Defects. If we can establish variations in bone density relating to types of failure in knee implants, then we still need to address various other issues. It is important to know whether the bone density changes are caused by the implant failing or are the cause of that failure. If they are the cause of the failure the possibility exists that there could be an underlying pre-existing difference in density (e.g. from medial to lateral) or that this arises secondary to the operation. One way in which the density differences could arise because of the operation would be the development of avascularity in the proximal part of the tibia due to the surgery. This 1429
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would be more likely on the medial side of the tibia as most incisions are medial and most surgical soft-tissue releases are medial. If such surgical insult were to result in a degree of osteonecrosis of the tibia, differential density changes would be likely. It is well described in other parts of the skeleton that avascular parts of the bone fail to participate in disuse osteoporosis and therefore density changes occur where the avascular part becomes more sclerotic and the surrounding bone porotic. This is the basis of Hawkins’ sign6 following talar neck fractures and also the early sclerotic reaction in the proximal pole of the scaphoid following scaphoid fracture and AVN of the proximal pole. One possible explanation of the findings of this article might therefore be that the surgery has damaged the medial tibia in a small number of cases, and where this occurs the density beneath the tibial implant remains constant, while that of the lateral half diminishes due to stress protection and ‘disuse’. The greater degree of porosis in those cases with instability, and the fact that it occurs both medially and laterally, could possibly be explained by a somewhat exaggerated disuse reaction in those cases because the patients, being aware of instability, may actually be using the knee less.
Clearly there is much still to find out and these are important areas for further investigation. It is hugely encouraging to think that we may be able to use routine follow-up radiographs for this purpose. As the authors mention, there is one further drawback of the proposed system in that it cannot give such accurate comparative density data about the pre-operative state of the bone as there is no implant present to standardise the measurements. If this can be overcome by standardised metallic markers that would be another step forward perhaps enabling a variety of large scale studies such as the relative merits of different materials and constructs of tibial implant.
References 1. Ritter MA, Davis KE, Small SR, Merchun JG, Farris A. Trabecular bone density of the proximal tibia as it relates to failure of a total knee replacement. Bone Joint J 2014;96-B:1503–1509. 2. Small SR, Ritter MA, Merchun JG, Davis KE, Rogge RD. Changes in tibial bone density measured from standard radiographs in cemented and uncemented total knee replacements after ten years’ follow-up. Bone Joint J 2013;95-B:911–916. 3. Walker PS, Hsu HP, Zimmerman RA. A comparative study of uncemented tibial components. J Arthroplasty 1990;5:245–253. 4. Garg A, Walker PS. The effect of the interface on the bone stresses beneath tibial components. J Biomech 1986;19:957–967. 5. Ralis ZA. Bone quality defect - a more significant factor than osteopenia in patients with fracture of the femoral neck. J Bone Joint Surg [Br] 1983;65-B:365. 6. Hawkins LG. Fractures of the neck of the talus. J Bone Joint Surg [Am] 1970;52A:991–1002.
THE BONE & JOINT JOURNAL
Can we identify and predict failure of knee implants from routine bone density measurements? T. J. Wilton From Royal Derby Hospital, Derby, United Kingdom
T. J. Wilton, FRCS, Consultant Orthopaedic Surgeon Royal Derby Hospital, Uttoxeter Road, Derby, UK. Correspondence should be sent to Mr T. J. Wilton; e-mail: [email protected] ©2014 The British Editorial Society of Bone & Joint Surgery doi:10.1302/0301-620X.96B11. 34982 $2.00 Bone Joint J 2014;96-B:1429–30.
In this edition readers will find an interesting paper concerning the quality and density of bone surrounding the tibial implant of total knee replacements.1 This paper is interesting for a number of reasons. Firstly, the authors have been able to identify bone density changes and follow them in a quantitative manner from the routine post-operative radiographs obtained in the orthopaedic clinic. Secondly, they have identified density changes which occurred differentially in their cases which developed failure of the implant. Finally, they were able to identify different changes in the tibial density depending upon the type of failure which subsequently occurred in their patients’ knees. The use of routinely available radiographs clearly offers some substantial advantages over the use of DEXA scanning for this purpose. Most health services would find the use of routine and multiple DEXA scans prohibitively expensive and cumbersome in this context, even as a research tool quite apart from their use as a routine monitoring system. The system these authors used for identifying the density changes used digitised radiographs and has been previously reported by them.2 The system is modified in this paper to compare the cases whose knees failed with case-matched controls and then the whole series of radiographs is followed to identify changes in density over time. Finally parts of the implant are also used to give an ‘absolute’ density standardisation. The findings suggest that in knees that do not fail, there is a gradual decline in bone density beneath the tibial component both medially and laterally for several years after operation. This is expected as it simply reflects the stress protection that is known to occur in bone around knee implants and which has been described in detail by Walker many years ago.3,4 Those knees that failed due to instability appeared to have still lower density but this was similar on medial and lateral sides of the
VOL. 96-B, No. 11, NOVEMBER 2014
knee and this was only statistically significant shortly after the surgery. The knees failing by medial collapse and subsidence of the tibia, and those which failed with progressive radiolucent lines, had increased in the relative density on the medial side of the tibia even by two months post-operatively, and this relative increase compared with the control knees continued for several years after surgery. The authors consider that these findings indicate that there is overloading of the medial side of the knee in those knees whose implants collapse in varus and those which develop progressive radiolucency and therefore the medial tibia undergoes increase in density according to Wolff’s law. In those which loosen, this increased density proves insufficient to prevent the overload of the bone so that collapse and loosening occur anyway. This is a plausible explanation, but perhaps not the only possible explanation. We would like to know whether bone density changes are responsible for the loosening and collapse of tibial components of TKR, though it is important to realise that bone density and bone strength are not synonymous. This is true in part because of the complex relationship between bone size and patient size, but has also been demonstrated by the work of Ralis5 on Bone Quality Defects. If we can establish variations in bone density relating to types of failure in knee implants, then we still need to address various other issues. It is important to know whether the bone density changes are caused by the implant failing or are the cause of that failure. If they are the cause of the failure the possibility exists that there could be an underlying pre-existing difference in density (e.g. from medial to lateral) or that this arises secondary to the operation. One way in which the density differences could arise because of the operation would be the development of avascularity in the proximal part of the tibia due to the surgery. This 1429
1430
T. J. WILTON
would be more likely on the medial side of the tibia as most incisions are medial and most surgical soft-tissue releases are medial. If such surgical insult were to result in a degree of osteonecrosis of the tibia, differential density changes would be likely. It is well described in other parts of the skeleton that avascular parts of the bone fail to participate in disuse osteoporosis and therefore density changes occur where the avascular part becomes more sclerotic and the surrounding bone porotic. This is the basis of Hawkins’ sign6 following talar neck fractures and also the early sclerotic reaction in the proximal pole of the scaphoid following scaphoid fracture and AVN of the proximal pole. One possible explanation of the findings of this article might therefore be that the surgery has damaged the medial tibia in a small number of cases, and where this occurs the density beneath the tibial implant remains constant, while that of the lateral half diminishes due to stress protection and ‘disuse’. The greater degree of porosis in those cases with instability, and the fact that it occurs both medially and laterally, could possibly be explained by a somewhat exaggerated disuse reaction in those cases because the patients, being aware of instability, may actually be using the knee less.
Clearly there is much still to find out and these are important areas for further investigation. It is hugely encouraging to think that we may be able to use routine follow-up radiographs for this purpose. As the authors mention, there is one further drawback of the proposed system in that it cannot give such accurate comparative density data about the pre-operative state of the bone as there is no implant present to standardise the measurements. If this can be overcome by standardised metallic markers that would be another step forward perhaps enabling a variety of large scale studies such as the relative merits of different materials and constructs of tibial implant.
References 1. Ritter MA, Davis KE, Small SR, Merchun JG, Farris A. Trabecular bone density of the proximal tibia as it relates to failure of a total knee replacement. Bone Joint J 2014;96-B:1503–1509. 2. Small SR, Ritter MA, Merchun JG, Davis KE, Rogge RD. Changes in tibial bone density measured from standard radiographs in cemented and uncemented total knee replacements after ten years’ follow-up. Bone Joint J 2013;95-B:911–916. 3. Walker PS, Hsu HP, Zimmerman RA. A comparative study of uncemented tibial components. J Arthroplasty 1990;5:245–253. 4. Garg A, Walker PS. The effect of the interface on the bone stresses beneath tibial components. J Biomech 1986;19:957–967. 5. Ralis ZA. Bone quality defect - a more significant factor than osteopenia in patients with fracture of the femoral neck. J Bone Joint Surg [Br] 1983;65-B:365. 6. Hawkins LG. Fractures of the neck of the talus. J Bone Joint Surg [Am] 1970;52A:991–1002.
THE BONE & JOINT JOURNAL
