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Precision and accuracy in computer-assisted total knee replacement

University of British Columbia

In this study, I report on the development and preliminary testing of a computer-assisted technique for total knee replacement (TKR) surgery. In TKR, poor limb alignment increases the chances of early implant failure or complications requiring revision surgery. Errors of 3° to 5° in varus/valgus have been shown to cause poor outcomes. Based on 7 published studies (753 knees), I estimate the standard deviation (SD) of varus/valgus errors resulting from the current technique to be 2.6°. My long term hypothesis is that affordable and clinically practical computer assisted techniques can substantially reduce alignment errors and thereby improve outcomes enough to justify the additional cost, and my specific aim in this study is to estimate what alignment precision can be reasonably expected using such techniques. I propose a system which would use optoelectronic tracking of the patient and the cutting guides to potentially allow adjustment of the guides to the desired orientation with greater precision than current techniques. The system would be completely passive and the surgeon would perform all cutting and other interventions by conventional means. It would also eliminate the use of intramedullary rods (potentially reducing the risk of fat emboli), would not require any additional preoperative procedures or imaging (such as CT scans) other than conventional TKR planning, and would not require any invasive procedures (such as insertion of bone pins) remote from the normal operating field.- The goal of the system would be to help surgeons, particularly those less experienced in TKR, to consistently achieve their intended alignment with little or no increase in invasiveness, operating time, or recurring costs. The capital cost of the additional equipment and software is projected to be about C$75,000. With the proposed system, the mechanical axis is located intraoperatively by tracking motions of the femur to compute the hip centre, selecting the desired knee centre at the distal femur with an optoelectronic probe, and either tracking motion of the foot or digitizing the ankle ^ malleoli with a similar probe. I designed and built a prototype non-invasive hip tracker to eliminate the need for a bone pin or immobilization of the pelvis during this procedure. For the ankle centre, I tested a similar non-invasive foot tracker to allow tracking of foot motion without bone pins. As an alternative to'tracking foot motion, I designed and built an ankle digitizing probe to robustly locate the midpoint between the malleoli. To start a cadaver testing program, I wrote MATLAB operating functions for the optoelectronic localizer system as well as routines to test and simulate the TKR procedure on cadavers. To estimate the improvement in alignment we can expect from the system, I did three studies: First, I tested precision and accuracy of locating the hip and ankle centres by doing repeated measurements on 2 embalmed cadavers and one fresh cadaver. Secondly, I tested the precision and robustness of the new ankle digitizing probe with 6 different operators and 8 cadavers. Finally, to estimate the orientation difference between the cutting guide plane and the cut bone plane, I measured 20 simulated TKR cuts made by 2 surgeons and 3 untrained operators in cadaver bone using conventional cutting techniques. At the 95% confidence limit of the results from these studies, standard deviation (SD) of overall varus/valgus alignment for the limited cadaver population studied is 1.1°. This is approximately twice as precise as the current technique as reported in the literature. Flexion/extension SD of the distal femoral cut and the posterior slope of the tibial plateau cut are each 1.2°, two to three times better than published estimates of current technique. Cutting errors (the orientation difference between the guide setting and the cut bone) account for 90 to 95% of this variance. Proximal/distal SD of the hip and ankle centres is about 2 mm, limiting the precision of specifying femoral and tibial lengths; alternately depth of resection can be set by conventional means. Rotational alignment has not been addressed at this stage, although repeated measures on one specimen show SD of 1.1° (at 95% confidence limit) of internal/external rotation when digitizing the transepicondylar axis. The system can be used in its current state to test the TKR procedure on cadavers, although a clinically acceptable hip tracker, optically tracked adjustable cutting guides, and a more complete user interface to the software should be developed to make future testing more representative of the complete clinical procedure. These first results must be considered as a pilot study until more cadavers are tested. Robustness of the hip tracker on different individuals has not been shown, and results could be considerably worse on individuals with indistinct iliac crests (primarily due to obesity). Although precision of the new ankle probe has been shown (318 measurements), accuracy of the resulting ankle centre with respect to the centre of area of the talocrural joint could only be measured on one specimen, and the relationship to a true kinematic centre has not been investigated. More surgeons must be included in the bone cutting test to fairly represent average TKR technique. Considering these limitations, I conclude that the proposed system has the potential to allow TKR alignment with SD of about 1 ° and that further development and testing is warranted.

Thesis/Dissertation

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2009-07-06

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http://hdl.handle.net/2429/10266

Mechanical Engineering

University of British Columbia

UBCV

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