The effect of medial unicompartmental knee replacements on the kinetic and kinematic parameters of the knee : the role of alignment and the effect of articular surface on regulation of medial compartment loading

Medial compartment loading is a significant factor in the progression of medial compartment osteoarthritis and medial unicompartmental replacement arthroplasty is a popular surgical treatment for medial unicompartmental knee osteoarthritis. However, there is no consensus on importance of alignment in medial unicompartmental knee replacement in orthopaedic literature. Static alignment measures like Hip Knee Ankle (HKA) angle and location of mechanical axis (MA) on the tibial plateau are known to affect loading of the knee. In addition, Peak knee adduction moment (PKAM) and angular adduction impulse (Add Imp) measured on instrumented gait analysis are recognised proxies for medial compartment loading especially in the coronal plane. A review of ninety four unicompartmental knee replacements revealed that survivorship at 10 years was 94% and at 15 years was 87%. Significantly better (p<0.01) function scores as measured by the 50 point Bristol Knee score was demonstrated in knees with good alignment where the mechanical axis passed through zones 0 or 1. An investigation into 18 medial unicompartmental knee replacements resulted in a statistically significant improvement in all domains of WOMAC score and the modified knee society score. Medial unicompartmental replacement arthroplasty resulted in a statistically significant improvement in gait velocity (p<0.01) and double limb support time (p <0.001). Further investigations into the kinetic and kinematic parameters showed excellent reliability. The coronal plane kinetics, peak knee adduction moments (ICC = 0.99), sagittal plane kinematics, flexion (ICC =0.98), sagittal plane plane kinetics, flexion extension moment (ICC =0.87), transverse plane kinematics, rotation (ICC=0.87) and transverse plane kinetics, rotation moment (ICC =0.76) showed excellent reliability as per Fleiss’s classification for intra class coefficients. Only coronal plane kinematics, varus valgus (ICC= 0.66) did not fall into the excellent category, but was still considered good. The coronal plane loading parameters improved significantly following medial unicompartmental replacement (p<0.05). The reduction in loading was related to the correction in alignment. The only sagittal plane kinetic parameter that improved significantly following medial unicompartmental replacement was the early stance extension moment (p ii! =0.05). The transverse plane kinetics, sagittal, coronal and transverse plane kinematics did not show statistically significant differences between the preoperative and postoperative groups.On single regression analysis, the mean adduction angle (MAA, p= 0.007) was a better predictor of coronal plane loading in the preoperative group, while the Hip Knee Ankle angle (HKA, p=0.01) was the better predictor in the postoperative group. As the adduction moments between individual knees was variable between knees even though normalised for height and weight, the percentage improvement in PKAM (%ΔPKAM) and Add Imp (%ΔAdd. Imp) was used for further analysis. On multiple regression, the effect of change of HKA (ΔHKA) was more significant (p = 3.5E-09) on %ΔAdd. Imp, than ΔMAA (p = 0.01). The correction in HKA was a significant predictor of improvement in the Add Imp (r2 = 0.90) and PKAM (r2 = 0.50). For every one degree correction of static alignment (HKA) a 7% improvement can be achieved in coronal plane loading (Add Imp). On comparison of coronal and transverse plane loading data between the asymptomatic non arthritic knees and replaced knees in this group of participants, the adduction moment curve, parameters like PKAM, Add Imp and rotation moments were nearly identical, implying that articular surfaces, does not seem to have a significant effect on the regulation of its own loading. The improvement of loading with correction of alignment is likely to have an impact on wear and subsequent survivorship of the prosthesis.