CORRELATION OF POSTERIOR FEMORAL OFFSET AND FLEXION IN TKR: STUDY OF 21 BILATERAL TKRs

Introduction

One of the important criteria of the success of TKR is achievement of the Flexion ROM. Various factors responsible to achieve flexion are technique, Implant and patient related. Creation of the Posterior condylar offset is one such important factor to achieve satisfactory flexion.

Aim

To correlate post op femoral condylar offset to final flexion ROM at 1 yr. post op.

Methods

This is a clinico-radiological study of the cases done prospectively between September 2011 and August 2012.

Inclusion criteria:

All patients undergoing Bilateral TKRs and have agreed for the follow up at 1 yr.

Exclusion criteria:

1. Patients who had previous bony surgery on lower end femur.

2. Patients with previous fracture of lower end femur.

All the patients had PS PFC Sigma (De Puy, Warsaw) components cemented. ROMs were measured at 6 weeks, 3 months, & 1 year post op. The last reading was taken as final flexion ROM as measured by a Physiotherapist with the help of a Goniometer.

Results

We had 21 cases of Bilateral TKRs who satisfied our criteria. Pre and post op femoral condylar offset was measured in mm. on lateral x ray. Pre and post op flexion was measured. Results showed that variation in the posterior femoral offset by > 3mm in post op x ray was related to loss of flexion of an average 21 deg. (16 – 24 degrees). Greater the deflection from the normal offset, greater was the loss of flexion. These patients also showed lesser improvement in KSS functional score.

Discussion

Flexion is one of the most important yardsticks for the measurement of success of TKR. This factor is more important more so in Asian population. Literature has shown that three important determinants for good flexion are….

1. Posterior Condylar Offset Restoration

2. Tibial slope restoration

3. Femoral Roll back in flexion.

An increased offset permits greater flexion before impingement between the tibial insert and the femur. In our study we kept Tibial slope and Femoral Roll back constant by using the same prosthesis. The femoral condylar offset changed as per the size of the AP femoral cutting block. (Anterior referencing guide used).

Overresection of the posterior condyles reduced the posterior femoral condylar offset and hence significant loss of post op flexion. The shorter posterior condyle of smaller femoral component can increase the potential for bone impingement proximal to the posterior condyles. In our study the opposite side replaced knee acted as a control. It is generally stated that after a TKR flexion can improve upto 1 year and hence was taken as final possible flexion.

Conclusion

Keeping Tibial slope and Femoral roll back constant during the surgery, posterior condylar offset restoration within 3 mm of its original pre op offset was necessary to achieve satisfactory flexion at 1 year. Undersizing the femoral component to achieve more flexion is perhaps suboptimal. Appropriate AP femoral sizing is a must to restore the normal offset.