[Modification of the Blackburne-Peel ratio for improved application in total knee arthroplasty].

BACKGROUND

The methods currently used to measure patella height in relation to the tibiofemoral joint line after total knee arthroplasty are not easily reproducible. For this reason we modified the Blackburne-Peel ratio.

MATERIALS AND METHODS

Instead of taking the tibial joint line as the reference line, we constructed a tangent on the femoral condyle. In addition, we took the greatest diameter of the patella from the tip of the patella as a measuring line.

RESULTS

Compared with the classic Blackburne-Peel and Caton-Deschamps ratios, it was much easier to determine the patella height in osteoarthritic knees and after total knee replacement using this modified Blackburne-Peel ratio. Factors such as the height of the polyethylene inlay, the slope of the tibial component, and superimposition of the patellar joint line can be disregarded. Preoperative and postoperative reference lines can be compared readily. Furthermore, the classic Blackburne-Peel ratio shows a greater tendency to interpret the measurements after total knee arthroplasty as pseudo-patella baja. The interobserver variability was good for all ratios.

CONCLUSION

We can therefore recommend the modified Blackburne-Peel ratio for measuring patella height in relation to the tibiofemoral joint line after total knee arthroplasty.