Computer assisted total knee arthroplasty (TKA) enables the measurement of the dynamics of the knee both before and after the implant of the prosthesis. Much time has been spent looking at the outcomes of navigated TKA however less time has been invested on understanding how the data collected pre-operatively can inform the surgeon and help the surgical decision making process. The aim of this work was to use navigation as a tool to quantify and classify preoperatively valgus knees.

Between August 2006 and September 2007 a group of 51 patients who demonstrated intra-operative initial neutral or valgus aligned knees underwent navigated TKA using the Columbus knee prosthesis and the Orthopilot^® navigation system (BBraun, Tuttlingen, Germany). Demographic data were recorded, along with the preoperative radiograph appearance and clinical assessment of alignment. During the surgery the approach used and the knee mechanical femorotibial (MFT) angle though the range of flexion were recorded. The knees were then categorised as either “True” valgus or “False” valgus based on whether the MFT angle at 30°, 60° and 90° flexion was still valgus (True) or had gone into varus (False).

Five patients were excluded from the study group as they had incomplete data in knee flexion. Of the remaining 46 patients, 28 were True valgus and 18 were False valgus. For the two groups demographic data were compared. Male to female ratio was 9:19 for the True valgus and 4:14 for the False valgus. The mean age of the True group was 70 years (range 52–85 years) and the False was 69 years (range 53–84 years). For BMI the True group had mean of 31 (range 20–40) and False of 33 (range 26–42). Twenty-five of the 28 True valgus knees showed preoperative evidence of clinical genu valgum deformity and radiologic evidence of predominantly lateral compartment osteoarthritis. Five patients had ipsilateral hip replacements in the past and five had rheumatoid arthritis. Seventeen were operated by lateral parapatellar approach. Eighteen required ilio-tibial band release with additional lateral collateral ligament release in five knees. Six true valgus knees did not require any soft tissue release. Five patients required lateral retinacular release to achieve thumb free patellar tracking. The median operating time for the True valgus group was 80 mins. Ten of the 18 false valgus knees showed evidence of clinical varus deformity and radiological evidence of predominantly medial compartment osteoarthritis. Only one patient had an ipsilateral hip replacement in the past and one had rheumatoid arthritis. All 18 knees underwent TKA by medial parapatellar approach, requiring no additional soft tissue release in 17 knees and a moderate release in one knee. The median operating time for the False valgus group was 60 mins.

True valgus knees had more significant deformities clinically and radiologically, longer surgical time and more incidence of soft tissue release when compared to the False valgus knees. False valgus knees behaved like varus knees clinically, radiologically and intra-operatively and should therefore be treated as such when making surgical choices.