Restoration of proper alignment is one of the principle goals of TKA. Various methods are popular, including intramedullary (IM) and extramedullary (EM) mechanical guides, and recently computer assisted navigation (CAS). In addition, minimally invasive surgery has added an extra level of complexity to achieving satisfactory alignment. The purpose of this study was to determine the effect of approach (standard arthrotomy vs MIS) and alignment technique (Mechanical vs CAS) upon component alignment in TKA.

Methods. Three consecutive cohorts of patients were included: Group I--Standard arthrotomy with Mechanical guides; Group II-MIS approach with Mechanical guides; Group III-MIS approach with CAS. A single surgeon performed the Standard Mechanical cohort, and a second surgeon performed all surgeries in the other two cohorts. For the mechanical groups, IM femoral and EM tibial guides were used. For CAS, the Orthosoft system was used. All components were NexGen (Zimmer) Postoperative x-rays were used to measure component alignment relative to the IM axes, including femoral valgus and flexion, and tibial varus and slope, and patellar tilt. In addition, joint line position was measured. Students’ t-test was used to determine level of significance.

Results. For Groups I, II and III, there were 41, 38 and 39 patients, respectively. For femoral alignment in the coronal plane, results were 4.83+4.29 degrees, 3.82+2.72 degrees, and 3.36+2.49 degrees, respectively. Femoral flexion was 2.93+2.82 degrees, 3.18+2.93 degrees, and 2.46+2.79 degrees, respectively. Tibial alignment was 0.44+3.98 degrees of varus, 1.00+2.83 degrees of valgus, and 0.95+2.58 degrees of varus, respectively. Slope was 6.78+3.23 degrees, 3.23+3.21 degrees, and 3.93+2.85 degrees, respectively. Patellar tilt was 2.15+3.51 degrees lateral, 1.73+2.67 degres lateral, and 1.03+2.28 degrees lateral, respectively. The joint line was raised 1.18+3.54 mm and 0.05+4.92 mm in Groups I and III, respectively, and lowered 0.33+4.78 mm in Group II. There were no statistically significant differences in any measurement between any groups.

Discussion. Satisfactory alignment can be achieved with either mechanical guides or navigation systems. MIS approaches do not worsen alignment with either alignment methodology. Whether having fewer outliers translates into improved clinical outcomes remains to be seen. More importantly, CAS provides an intraoperative tool that may allow more accurate reproduction of a customized plan for an individual, rather than simply attempting to achieve the “mean” for a population. Again, the value of achieving such a goal is unknown since the threshold for improvement with off-the-shelf knee components may have already been maximised.