Inaccurate positioning of components results in suboptimal knee function, implant wear and early loosening of the prosthesis. Small differences in varus/valgus angulation have been shown not to affect outcome, but, when the prosthesis is more than 3 degrees malaligned from neutral, premature failure rates rise.

Accurate alignment requires proper orientation in the placement of the cutting guides and computer-aided navigation systems have now been developed with the aim of improving this.

We compared the post-operative leg alignment following computer assisted (Orthopilot) versus conventional (IB2 with extramedullary tibial and intramedullary femoral jigs) methods of TKJR using weight-bearing long leg radiographs.

This was a study of 91 consecutive TKJRs (51 IB2s and 40 Orthopilot Search Evolution knees) performed in 70 patients.

A single experienced knee surgeon carried out all these procedures. All these patients had weight-bearing long leg alignment films taken by a single experienced radiographer.

The anatomical centres of ankle, knee and hip were then marked on each film and the tibia femoral angles drawn. Two separate blinded observers then measured the angles of malalignment.

Observer A’s results show that 95% of the Orthopilot knees were within 3 degrees of varus or valgus from neutral. Only 74.47% of the IB2 group were within this range (p=0.011).

For Observer B 87.5% of Orthopilot knees and 70.21% of IB2s were within the range (p=0.052). When we group these finding we see that an average of 91.25% of the Orthopilot and 72.34% of the IB2 knees are within the range (p=0.025).

When the interobserver figures for each group were compared no significant difference was found.

Conclusion: Our results show a significant improvement in postoperative alignment of TKJRs by using the computer-aided navigation system and it should follow that the long-term survival of the prosthesis would be extended.

Introduction and Aims: Accurate alignment of TKJRs is essential for the survival and success of the procedure. Intra-operative alignment of implants is limited by the accuracy of the instrumentation and conventionally relies on visual inspection for confirmation. Computer-aided navigation systems have been developed with the aim of improving component alignment and thus the success of TKJRs. One method of accurately checking post-operative alignment of knee prosthesis is weight-bearing long leg films.

Method: We compared the post-operative leg alignment following computer-assisted versus conventional methods of TKJR using weight-bearing long leg radiographs.

In 91 consecutive TKJRs, the first 50 were performed by conventional methods and a computer navigation system was used for the last 41 in the series. All were performed by the same experienced knee surgeon.

The tibiofemoral degree of malalignment was measured in each long leg film, using the centre of the hip, knee and ankle joints as reference points. Two observers performed each measurement blind and at a single sitting. The same experienced radiographer took all the x-rays in a standardised manner using a kodek long leg film and cassette.

Results: The results show that accuracy of alignment was significantly improved by the computer-assisted navigation system with 91.25% of the Orthopilot and 72.34% of the IB2 knees being within three degrees of varus or valgus from neutral (p=0.025).

Conclusion: These results show a significant improvement in post-operative alignment of caps TKJRs by using the computer-aided navigation system and should follow that the long-term survival of the prosthesis would be extended. We suggest that a more universal analysis of alignment in TKJR is required, as well as a long-term follow-up of patients with post-op alignment measurements.