gms | German Medical Science

Objectives: Osteoarthritic disease progression accommodated by coronal malalignment of the lower limb may distort the geometrical relationships of the native knee. The objective of this study was to investigate the relationship between the femoral and tibial joint line orientation with an increase in coronal varus deformity using three-dimensional (3D), computer-aided design (CAD) bone models.

Methods: One hundred fifty (150) 3D-CAD-bone models of patients with varus osteoarthritis who had been scheduled for customized TKA were analyzed. Patients were retrospectively selected based on the hip-knee-ankle angle (HKA) and allocated to one of three groups of mild (0-5°; n=50), moderate (>5-10°; n=50) and severe (>10°; n=50) varus deformity. Joint line orientation was measured based on the femoral and tibial mechanical angle (FMA and TMA), defined as the medial angle between a tangent to the distal femoral condyles and proximal tibial plateau and the femoral and tibial mechanical axis, respectively. The joint line congruency angle (JLCA), defined as the angle between the tangents to the articulating surfaces of the femur and tibia, was computed. Lastly, a previously introduced concept of functional knee phenotyping based on the orientation of the tibial and femoral joint lines was applied to this population of varus osteoarthritic knees. A standard two-tailed t-test assuming unequal variances was used to determine significant differences between the groups.

Results: Average HKA were 176.7° (SD, 1.2), 172.6° (SD, 1.4) and 167.9° (SD, 1.1) in groups of mild, moderate and severe varus, respectively. Average FMA values decreased from 92.3° (SD, 1.9) to 91.4° (SD, 1.5) (p=0.01) and further to 89.5° (SD, 2.1)(p< 0.0001) from mild to severe varus. Average TMA values decreased from 86.6° (SD, 2.2) to 84.6° (SD, 2.1)(p< 0.0001) and further to 82.9° (SD, 1.9) (p= 0.0002) in the three groups of mild, moderate and severe varus, respectively. The JLCA increased significantly with an increase in HKA from an average of 2.2° (SD, 1.7) to 3.5° (SD, 1.8) (p= 0.0003) and 4.5° (SD, 1.9) (p= 0.007) in the three groups, respectively. The distribution of functional knee phenotypes revealed that tibial deformity contributes predominantly to overall limb malalignment with an increase from mild to moderate varus deformity whereas an increased femoral joint line obliquity was observed in cases of severe varus deformity.

Conclusion: Results of this study demonstrate that tibial deformity is dominant in mild varus cases, femoral and tibial deformity contribute equally to the degree of overall malalignment in cases of severe varus. Although joint laxity plays an increasing role with an increase in severity of lower limb malalignment, an increasing potential for intra-articular alignment correction is also observed when cuts in relation to the mechanical axis are performed. This should be taken into account when considering options for lower limb realignment such as osteotomy or total knee arthroplasty.