Introduction

There is little scientific evidence on the effect of knee joint line obliquity (JLO) before and after coronal re-alignment osteotomy. Bony alignment of the knee joint is an important factor in its normal function, different pathologies, and load distribution. There is little evidence on how coronal JLO affects joint pressure, shear forces, and joint movement when weight bearing. There is no clearly accepted JLO cut-off defined, to facilitate the indication of either single-level or double-level osteotomies. It is generally accepted that 10° of JLO is the arbitrary upper limit accepted for osteotomy surgery.
Hypotheses: It was hypothesized that higher JLO would lead to abnormal relative position of the femur on the tibia, a shift of the joint contact areas, and elevated joint contact pressures.
Study design: Descriptive Laboratory Study

Methods

10 fresh-frozen human knees (age 59 ± 5 years) were axially loaded to 1500N in a materials testing machine with the joint line tilted 0°, 4°, 8° and 12° varus (‘downhill’ medially) and valgus, at 0° and 20° knee flexion, simulating weight-bearing gait. The mechanical compression axis was aligned to the center of the tibial plateau. Contact pressure and contact area were recorded by pressure sensors inserted between the tibia and the menisci. Changes in relative femoral and tibial position in the coronal plane were obtained by an optical tracking system. Data were analyzed by repeated-measures ANOVA with post-testing.

Results

Both medial and lateral JLO caused significant tibiofemoral subluxation and pressure distribution changes. Medial (varus) JLO caused the femur to sublux medially down the coronal slope of the tibial plateau, and vice-versa for lateral (valgus) downslopes (P<0.01), giving 6mm range of subluxation. The areas of peak pressure moved more than the bone subluxations, by 12mm and 8mm across the medial and lateral condyles, onto the ‘downhill’ meniscus and the ‘uphill’ tibial spine. The loaded meniscus acted as a sling to resist further subluxation. Changes in JLO had only small effects on mean and maximum contact pressures.

Conclusions

A 4° change of JLO during load bearing causes significant mediolateral tibiofemoral subluxation. The femur slides down the slope of the tibial plateau to abut the tibial eminence and also to rest on the ‘downhill’ meniscus. This causes large movements of the tibiofemoral contact pressures across each compartment.
Clinical relevance: These results provide important information for understanding the consequences of creating coronal JLO, and for clinical practice in terms of osteotomy planning regarding impact on JLO. It provides guidance regarding the choice of single- or double-level osteotomy. Excessive JLO alteration may cause abnormal tibiofemoral joint articulation and chondral / meniscal loading.
What this study adds to existing knowledge: This study provides biomechanical in-vitro data on the effect of changing JLO on tibio-femoral subluxation and pressure distribution changes. Furthermore it shows how the menisci and tibial eminences resist movements of the femur in the coronal plane in abnormal JLO along with an increase in stress at these areas. This is important to consider when performing osteotomy especially in the athletically active patients.