A new method for evaluation of patellar height and the position of the joint line before and after total knee arthroplasty

Abstract

Background: The measurement of patellar height and restoration of the natural position of the joint line are crucial to total knee arthroplasty (TKA). However, there remains a lack of consensus on an optimal measurement method to associate the patellar height with the joint line position. The objective of this study was to introduce a new method and validate the application in TKA both preoperatively and postoperatively.

Methods: Instead of taking marginal landmarks as the tibial references, the tibial shaft axis was used to construct the new measurement method, which comprises the axis-patella (AP), joint axis-patella (jAP) indices and joint line height (JLH). Patellar heights were measured using the Insall-Salvati (IS), modified Insall-Salvati (mIS), Blackburne-Peel (BP), Caton-Deschamps (CD) indices, and the new method in 175 knees both preoperatively and postoperatively. Intraclass correlation coefficients and Pearson's correlation analyses were respectively used to evaluate the reliabilities and correlations.

Results: There were good correlations between the proposed method and the mIS, CD, and BP indices. High inter-observer reproducibility was found for AP (preoperative and postoperative 0.83), jAP (preoperative 0.82; postoperative 0.86) indices and JLH (preoperative 0.88; postoperative 0.95). High intra-observer repeatability was also found for AP (preoperative 0.85; postoperative 0.87), jAP (preoperative 0.83; postoperative 0.87) indices and JLH (preoperative 0.80; postoperative 0.92).

Conclusions: The new method is reliable for measuring patellar height before and after TKA, providing an alternative to distinguish between true and pseudo patella baja. Furthermore, JLH can be applied to assess and restore the joint line position in TKA.

Keywords: Joint line position; Patella baja; Patellar height; Reliability; Total knee arthroplasty.

Fig. 1

Representation of the four classical methods used for measuring patellar height. As represented in A and B (preoperative and postoperative radiographs, respectively), the Insall-Salvati index is calculated by “a1/b1”, and the modified Insall-Salvati index is calculated by “a2/b2”. As represented in C and D (preoperative and postoperative radiographs, respectively), the Caton-Deschamps index is defined as “a1/b”, and the Blackburne-Peel index is defined as “a2/b”

Fig. 2

Illustration of the new method proposed in this study. The axis-patella (AP) index is defined as “a1/b”. Measurement “a1” is measured from the lower pole of the patellar articular surface to the intersection point (T1) between the tibial shaft axis and its perpendicular line passing through the tip of the fibular head. Measurement “b” is the length of the patellar articular surface. The joint axis-patella (jAP) index is calculated by “a2/b”. Measurement “a2” is measured from the lower pole of the patellar articular surface to the intersection point (T2) between the tibial shaft axis and the joint line. The joint line height (JLH) is defined as the distance between the T1 and T2

Fig. 3

The scatter chart exhibiting the change trend of the joint line height in 175 cases both preoperatively and postoperatively

Fig. 4

Bland-Altman plots of the preoperative and postoperative differences for measuring patellar height between twice measurements by the same author with a 3 months interval: the axis-patella (AP) index (A and C, respectively), the joint axis-patella (jAP) index (B and D, respectively)

Fig. 5

Bland-Altman plots of the preoperative and postoperative differences for measuring patellar height between two authors: the axis-patella (AP) index (A and C, respectively), the joint axis-patella (jAP) index (B and D, respectively)