The vastus medialis obliquus insertion: a classification system relevant to minimally invasive TKA

When performing minimally invasive total knee arthroplasty, the level of the vastus medialis obliquus (VMO) insertion is important, as a distal insertion may prevent a true quadriceps-sparing approach. We have previously reported a variable VMO insertion to the patella. In this article we propose a simple classification system to describe the level of insertion of the VMO, the purpose of which is to enable accurate documentation during minimally invasive approaches to the knee. Recent work by this group identified that the mean distal insertion point of the VMO was 51% of patellar length (range, 13%-95%, SD 13.9%). There was a Gaussian distribution of insertion around the mean value; however, 25% of individuals were found to have a distal insertion >60% of patellar length. As such we propose a simple classification to document both the level of insertion and extent of incision during minimally invasive total knee arthroplasty. We have divided the medial border of the patella into 3 equal segments. The majority of individuals will have a type II patella. A patient with a type III VMO would be unsuitable for a quadriceps-sparing approach without incising the distal fibers of the VMO. The clinical effects of extending the incision in this way are not known; however, if we wish to evaluate this scientifically, we must have a way of documenting both VMO insertion and the level to which we have incised the extensor mechanism. This classification allows simple documentation of these variables.