Abstract
Introduction: The advent of digital radiological technology affords orthopaedic surgeons with the opportunity to better assess prosthetic alignment. The purpose of this study was to determine whether a post-operative digitalised 3-foot standing film better helps the surgeon than the standard weight-bearing short analogue antero-posterior film.
Materials and Methods: A single surgeon’s series of 68 consecutive patients were included in this prospective study. The axial alignment, mechanical, anatomical axes and distance of the post-operative mechanical axis from the centre of the knee, of 71 total knee replacements were determined using computer software on digital 3 feet standing films (Agfa ADC-2S system). These results were compared to similar measures determined using a goniometer and standard short standing analogue antero-posterior films. Both sets of results were compared to the intra-operative distal femoral valgus cut used by the surgeon.
Results: Sixty-eight patients, 71 knees, average 68.1 (range 42–85) years were included in this study. The average length of time from surgery to complete radiological evaluation was 18.9 (range 2–40 months). A mean difference of 1.20 degrees was found between the valgus angles measured on digital long and standard short radiographs. This difference was statistically significant p< 0.05. Comparing both sets of films revealed a Pearson correlation co-efficient of 0.65. Comparing the valgus angle determined on the digital 3 foot standing film to the intra-operative valgus cut revealed a Pearson correlation co-efficient of 0.388, this figure compares to a correlation co-efficient of 0.240 when the standard short films are used. The postoperative mechanical axis was found to be on average 7.34 (range 0.0 – 30.6) mm from the centre of the knee and the implants were positioned on average 89.8° to the mechanical axis on digitalised films, both these measures were indeterminable on the short standard films.
Conclusion: Three-foot digital standing antero-posterior films are more accurate and better correlate with the surgeon’s pre-operative aims than standard short antero-posterior standing radiographs. Only digital films allow for accurate determination of the post-operative mechanical axis. Despite not using expensive computer aided navigation systems intra-operatively, implant positioning was very satisfactory. Short films are adequate for routine assessment of total knee replacements postoperatively but digital 3-foot standing films afford better accuracy and are preferable for research purposes.
The abstracts were prepared by Emer Agnew. Correspondence should be addressed to Irish Orthopaedic Association, Secretariat, c/o Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
