Midline skin incision and medial arthrotomy for knee arthroplasty may be preformed in flexion or extension. Anatomical studies have revealed a risk to the infra patellar branch of the saphenous nerve. This study addresses

whether sensory loss is greater following skin incision in flexion or extension.

Null hypothesis- there is no detectable difference in sensation before and after knee arthroplasty. Pilot study- light touch, sharp touch and two point discrimination were evaluated pre-operatively, at one week, six weeks and six months postoperatively in twelve patients recruited prospectively. Randomization was achieved using an envelope system. Six patients underwent approach and closure in flexion and six in extension. Prospective cohort study – 50 patients underwent sensory mapping for light touch and sharp touch pre-operatively. These control results were compared with the post-operative findings at six months. All measurements were standardised to anatomical landmarks with the knee in 90 degrees flexion.

There is a constant area of sensory loss lateral to the midline scar, which shows some recovery with time. The sensory loss affects both light and sharp touch. Initially, this is in a similar distribution anteriorly and laterally extending from the superior pole of the patella to the tibial tubercle, approximately 2cm lateral to the midline. The loss is most marked immediately after surgery. There is a noticeable recovery in sharp touch by six weeks. The recovery in light touch is slower and less complete at the six month review. There is no demonstratable difference in sensory loss regardless of whether the incision is made in flexion or extension, (p=0.1)

Lateral sensory loss is a constant feature in this series six months following knee arthroplasty. Patients may benefit from pre-operative counselling regarding the likelihood of lateral cutaneous sensory deficit following knee arthroplasty.