The Effect Of Limb Alignment On The Clinical Outcome Of Mobile Bearing UKA In Patients With Osteoarthritis: A Retrospective Study

Background Limb alignment is one of the factors that tend to affect patient outcomes after medial unicompartmental knee arthroplasty.Excessive varus alignment has been shown to put a strain on the UKA bearing causing implant related complications like loosening,wear of the polyethylene and recurrence of deformity by concentration of forces on the medial knee compartment.Valgus alignment on the other hand causes contralateral joint disease progression in addition to poor function and dislocation of bearing.Previous studies have largely been inconclusive in reporting this relationship between alignment and clinical outcome.Some studies have found a strong relationship,some have found no association and yet in some there is only a relationship with revision rates.The effect of the preoperative limb alignment on clinical outcome on the other hand has not been studied before.The understanding of the effect of limb alignment on clinical outcomes will assist us to optimize patient selection by identification of patients at risk of poor outcomes and also help us determine the appropriate limb alignment we should be aiming at during UKA surgery.Therefore in this study for my first objective,I first sought to determine the postoperative limb alignment outcomes after UKA at our institution.Secondly I sought to determine the effect of the preoperative varus deformity on the postoperative limb alignment.Lastly I sought to determine how both the preoperative varus deformity and postoperative limb alignment influenced the clinical outcome as measured by the hospital for special surgery(HSS)knee scores after 2 years of follow-up.Materials/patients and methods: Before operation,limbs were divided into 3 groups according to the severity of the preoperative varus deformity.Group 1 had limbs with a severe preoperative varus deformity(HKA=165-170°),group 2 had limbs with a moderate preoperative varus deformity(HKA=170-175°)andgroup 3 had limbs which had a mild preoperative varus deformity(HKA=175-180°).Patients were then followed up after operation and according to the postoperative limb alignment outcome the same limbs were again divided into three groups;those that remained in postoperative excessive varus alignment(HKA≤174°),those with the acceptable postoperative mild varus alignment of 177±3° and those that were overcorrected into valgus alignment postoperatively(HKA≥180°).The mean postoperative limb alignment and proportions of limbs in the three preoperative groups were evaluated to determine which limbs were likely to end up with excessive varus,mild varus or be overcorrected into valgus alignment postoperatively.After 2 years of follow-up,clinical assessment was then done using the hospital for special surgery(HSS)knee rating score to determine the effect of preoperative varus deformity and postoperative limb alignment on clinical outcomes.Results: The mean preoperative HKA significantly improved from 172.9±3.6° to a mean postoperative HKA of 176.7±2.9°(p<0.001)after operation.The difference in the mean postoperative HKA for the different groups based on the preoperative HKA was statistically significant(p<0.001).In 78.1% of limbs,the postoperative limb alignment was restored back to the expected mild varus alignment,12.5% of limbs remained in excessive varus and 9.4% of limbs were overcorrected into valgus.Greater proportion of limbs remained in excessive varus when the preoperative deformity was high and ended up in valgus when the preoperative deformity was low.The difference in proportion of limbs based on postoperative HKA in the different preoperative HKA groups was statistically significant(p=<0.001).The preoperative HKA and postoperative HKA were also strongly positively correlated(r=0.678)p<0.001)and the regression equation was y=78.294+0.568 x.The patient median HSS score significantly improved from 58 before operation to 93 two years after operation(p=<0.001).Limbs that were overcorrected into postoperative valgus alignment showed significantly lower 2 year median HSS scores as compared to the postoperative excessive varus and mild varus alignment groups.(84 as compared to 92.5 and 93 respectively)p=0.014).Similarly only 44.4% of the knees in the postoperative valgus group had an excellent outcome as compared to 83.3% and 88% in the postoperative excessive varus and mild varus alignment groups respectively(p=0.004).At the same time,the severity of the preoperative varus deformity had no effect on the clinical outcomeas no significant differences in the 2 year HSS scores were noted among the three preoperative deformity groups.Conclusion: From this study,majority of our patients had their limb alignment corrected back to the expected mild varus postoperative limb alignment of 177±3°.The preoperative limb alignment only had an effect on the postoperative limb alignment but no effect on clinical scores.On the contrary,the postoperative limb alignment had a statistically significant effect on our patient clinical and functional outcomes.Patient with postoperative valgus alignment had significantly lower scores than patients with mild varus and excessive varus alignment.Although the effects of excessive varus alignment on patient outcome were not apparent at 2 years of follow-up,they could be seen with continuous and long term follow-up.Our results suggest that an appropriate limb alignment should be pursued after UKA surgery.Surgeons should aim to avoid overcorrection into valgus at all costs in order to guarantee superior functional outcomes.This will reduce complications,minimize the frequency of revision and optimize patient care after medial UKA.