Abstract
The positioning of components in knee replacement is related to outcome and for this reason a study has been conducted to compare the exact position of the tibial and femoral components in total knee arthroplasty with the placement as judged by the surgeon at the time of operation.
Operating surgeons of a range of grades completed a pro-forma immediately after operating on 25 patients having total knee replacement. Patients were entered into the study by consent providing that they had osteoarthritis and this was their first ever lower limb joint replacement. The form detailed where the surgeon considered he had placed the femoral component in the coronal plane and in terms of rotation upon the femur. They were asked to state what lines or angles of reference they had used and whether they had used intra or extra medullary jigs. Likewise for the tibia, implant position was detailed for coronal, sagittal and transverse planes. The proforma stated the grade of operating surgeon but were otherwise kept anonymous.
All study patients had pre and postoperative CT scans. These involved an AP scannogram and transverse sections, according to a protocol, through the femoral neck, femoral condyles, tibial plateau and ankle. By comparing bony landmarks seen on the pre-operative CT scans with lines of reference from the components post-operatively the exact position of the implant was determined in the transverse and coronal planes. For the sagittal plane (slope) the standard lateral X-ray was used.
For the femur all operations were carried out using intra-medullary jigs. For the femoral component the difference was not significant between the measured position and the surgeons estimate in any plane (p=0.937 for coronal and p=0.432 for transverse). The measured position of the component was not related to the grade of the operating surgeon nor to the axis nor technique of reference used.
For the tibial component, coronal alignment was significantly different (p=0.001) with the measured position being in more varus than was estimated. The range of transverse placement was from 4° of external rotation to 35° of internal rotation of the tibial prosthesis with reference to the tibial tubercle centre. This was significantly different to that estimated by the surgeon (p< 0.001). Estimation of slope in the sagittal plane was good. None of these differences were related to operator grade. For 15 of the TKR’s the tibail component was aligned using intra-medullary techniques. This was related to the accuracy of positioning of the prosthesis with significantly better estimation compared to those in which extra-medulary jigs had been used (p=0.002 for the transverse plane and p=0.065 for the coronal plane).
This study has demonstarted that surgeons are able to accurately judge the position of insertion of the femoral component in total knee replacement. Surgeons are poor at estimating the position of the tibial component in the transverse and coronal planes but better in the sagittal plane. Due to the difficulty in its assessment rotational alignment has been ignored in arthroplasty but as with alignment in the other planes it is likely to have a bearing on outcome. Improved techniques to help us judge placement of knee components are needed.
The abstracts were prepared by Mr R. B. Smith. Correspondence should be addressed to him at the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
