Changes in the Contact Stress Distribution Pattern of the Patellofemoral Joint After Medial Open-Wedge High Tibial Osteotomy: An Evaluation Using Computed Tomography Osteoabsorptiometry

Abstract

Background: Medial open-wedge high tibial osteotomy (OWHTO) theoretically causes distalization and lateralization of the tibial tuberosity and the patella.

Purpose/hypothesis: The purpose of the study was to identify any changes in the stress distribution of subchondral bone density across the patellofemoral (PF) joint before and after OWHTO through the use of computed tomography (CT) osteoabsorptiometry. We hypothesized that OWHTO would alter the distribution of contact stress in the PF joint.

Study design: Case series; Level of evidence, 4.

Methods: A total of 17 patients (17 knees) who underwent OWHTO were enrolled in this study between September 2013 and September 2015. All patients underwent radiologic examination preoperatively and at 1 year postoperatively, and the distribution patterns of subchondral bone density through the articular surface of the femoral trochlea and patella were assessed preoperatively and >1 year postoperatively using CT osteoabsorptiometry. The quantitative analysis of the obtained mapping data focused on location of the high-density area (HDA) through the articular surface of the PF joint. The percentage of HDA at each divided region of the articular surface of the femoral trochlea and the patella was calculated.

Results: In the radiologic evaluation, the Blackburne-Peel ratio was significantly reduced (P < .001) after surgery, and the tilting angle of the patella was significantly decreased (P < .001). On CT evaluation, the percentage of HDA in the lateral notch and lateral trochlea of the femur and in the medial portion of the lateral facet of the patella increased significantly after OWHTO surgery (P ≤ .038).

Conclusion: OWHTO significantly increased the stress distribution pattern of the lateral trochlea of the femur and the medial portion of the lateral facet of the patella. The procedure significantly lowered the patellar height and significantly decreased the patellar tilting angle after surgery.

Keywords: high tibial osteotomy; knee osteoarthritis; medial open-wedge; patellofemoral joint.

Figure 1.

Flowchart for patients with inclusion and exclusion criteria. OWHTO, open-wedge high tibial osteotomy.

Figure 2.

(A) The subchondral bone region of the patellofemoral articular surface of the distal femur is automatically identified using our original software. In each axial slice, the Hounsfield units of the identified region are measured at each coordinate point. (B) Distribution of the subchondral bone density is determined by stacking the data obtained from axial slices. (C) For quantitative analysis, the distribution pattern is represented as a surface mapping image depicted by 8-grade color scale. Min, minimum; Max, maximum.

Figure 3.

Divided regions of the (A) trochlea and (B) patella for quantitative analysis of the obtained mapping data. CR, central ridge; LLF, lateral portion of the lateral facet; LN, lateral notch; LT, lateral trochlea; MF, medial facet; MLF, medial portion of the lateral facet; MN, medial notch; MT, medial trochlea.

Figure 4.

The mechanism whereby the compressive force on the lateral side of the patella is increased after open-wedge high tibial osteotomy (HTO). Lateral and distal shift of the tibial tubercle increased the pressure between the lateral patellar facet and the lateral wall of the trochlea (yellow allows) and forced up the lateral groove (red circle), leading to a medial tilt of the patella (red arrow).