Total knee replacement using the measured resection technique: do we get it right by accident?

Introduction The following question is often asked: why does the measured resection technique result in a balanced total knee replacement despite measuring from an eburnated joint surface distally and a cartilage-covered condyle posteriorly. The presented hypothesis is that the distal femoral bone-cut is distalised on average by 2 mm when measured relative to the medial condyle. This is due to the change in the distal condylar angle of approximately 3° and results in a differential distal bone-cut. This 2 mm distalisation of the femoral component compensates for using the cartilage-covered, posterior condyle as a reference point when using the measured resection technique. This article hypothesises why the measured resection technique works in total knee replacement. Conclusion Although the hypothesis is supported, implementing it into clinical practice is yet to be observed and requires further research. Introduction Two techniques are employed when implanting a total knee replacement (TKR), that of gap-balancing and measured resection1. In the measured resection technique, the remaining joint surface is used as a reference point from which the positions of the bony cuts are calculated. There is a vast amount of literature addressing the rotation of the femoral component with regard to the transepicondylar line, as rotation of the component affects stability in flexion and the kinematics at the tibiofemoral and patellofemoral joints2,3. However, there has been minimal attention given to the effect of measured resection upon gap-balancing. If it is accepted that the distal femoral bone-cut is measured relative to the eburnated surface, due to the full thickness loss of the cartilage and hence the indication for the TKR, then this must be different from the posterior condylar measure. When the posterior condylar bone-cuts are measured, it is not done in relation to the bony eburnated surface, but the preserved cartilage-covered surface due to minimal wear in this relatively non-weight bearing aspect of the knee. A recent study of osteoarthritic knees demonstrated that the cartilage over the posterior condyles was preserved and measured approximately 2 mm in thickness4. Also, this 2 mm measurement is consistent with the cartilage thickness in asymptomatic knees5. Hence, if it is accepted that the posterior condylar bone-cuts are measured relative to the cartilage-covered posterior condyles, in contrast to the eburnated distal condyle, then there will be at least a 2 mm difference in the extension and flexion gaps when using the measured resection technique. This should result in a loose extension gap or a tight flexion gap, but this does not seem to occur with good functional results that are equal to that of the gap-balancing technique6. This article presents a hypothesis on why the measured resection technique works and results in a balanced TKR, and discusses the potential implications if the parameters of this hypothesis are not observed. The anatomy of the distal femur is discussed first to support the hypothesis. We have also discussed how the anatomy of the distal femur results in a differential distal femoral bone-cut that matches the posterior condylar bone-cut made relative to the cartilage -covered surface. Th e distal femoral condylar angle Coronal alignment of the femur can be measured from the angle formed between a tangential line across the distal femur and the anatomical axis of the femur, which is termed as the distal condylar angle (Figure 1)7. The mean value of this angle is 81° with no significant differences with respect to gender or disease type8,9. Due to the current convention of making the bony tibial cut at 90° to the anatomical axis of the tibia, in the coronal * Corresponding author Email: [email protected] Department of Orthopaedics and Trauma, The Royal Infirmary of Edinburgh, Little France, Edinburgh, United Kingdom Figure 1: Distal condylar angle (X) formed by a tangential line across the distal femur and a line marking the anatomical axis of the femur, which has a mean value of 81°. Tr au m a & Or th op ae di cs