Computer-assisted navigation during total knee replacement has been advocated to improve component alignment and hence reduce failure rates and improve quality of life. The technique involves the placement of trackers via pins placed in both the femur and tibia throughout the surgery. It has been proposed that complication rates are higher in knee arthroplasty when computer navigation is used, compared to when it is not, due to increased risks from the pin tracker sites. Potential risks from pin sites include infection, fractures of the tibia or femur and pin site pain. In this study we present the post-operative complication rates related to pin tracker sites of computer navigated knee arthroplasty from a single surgeon at one centre. A database was compiled including all patients undergoing knee arthroplasty with computer navigation between January 2009 and December 2013 performed by a single surgeon at one centre. A retrospective study was undertaken having identified a total of 321 patients (642 pin sites) with 287 having undergone total knee replacement, 29 Uni-condylar knee replacement and five having undergone patellofemoral knee replacement. There 131 males and 190 females with a mean age of 69.4 [range 48–89]. There were no exclusions. The patient's notes were reviewed for any complications that occurred as a result of pin sites including infection, pin site pain and fracture. Only one patient (0.03%) was identified with a superficial pin site infection that was successfully managed with oral antibiotics only. There were no fractures or other complications identified in any of the other patients. In this series, the complication rates resulting from pin tracker sites was very low suggesting computer navigation does not increase the risks of knee arthroplasty. There were no cases of femoral or tibial fractures in this series, as have previously been reported. It is therefore likely that the technique of pin site placement is important in limiting the risk of complications. In this series a standard technique was used in all cases. Stab incisions are always used rather than a percutaneous technique and the wounds closed with clips and protected with dressings at the end of the surgery. Uni-cortical drilling is sufficient to provide stability of the trackers intra-operatively and minimises the risk of thermal necrosis therefore bi-cortical placement is avoided. Self-drilling pins are used on power and inserted perpendicular to the bone on high torque and low speed. The tourniquet is not inflated until after the pins have been inserted. It is thought that using this technique offers a safe method of pin tracker placement ensuring low complication rates.
Oxford medial uni compartmental knee replacement is a common and widely accepted procedure that relies on accurate positioning and alignment of the implants for optimal outcome and longevity. Posterior slope of the tibial base plate has been shown to be an important factor affecting long term survivorship. The aim of the study was to evaluate whether navigation increased the accuracy of Oxford knee replacements using the posterior slope of the tibial component as an index measure. The posterior slope of tibial trays from 58 sequential Oxford medial unicondylar knee replacements over a two year period was checked on standard lateral x-rays against the recommended range. There were 12 cases in the navigated and 46 in the conventional group across six Orthopaedic firms. The mean posterior slope for navigated and conventional implantations was 4.75 and 3.3 degrees respectively with the difference not being statistically significant. However, when considering the data for low volume surgeons, the mean posterior slope with and without navigation was 4.75 and 1.83 degrees respectively which was significant with a p value of 0.017. Navigation was also found to significantly decrease the chance of implanting the knee with the posterior slope outside the acceptable range (p=0.024). In both analyses the navigated cohort had a narrower data spread and fewer outliers compared to the conventional group. No other factors were found to significantly correlate with the posterior slope. The study suggests that navigation might help low volume surgeons in increasing the accuracy and decreasing the incidence of extreme variations from the desirable range of implant positioning for unicompartmental knee replacements.
Optimal soft tissue tension maximises function following total knee arthroplasty. Excessive tension may lead to stiffness and or pain, while inadequate tension can lead to instability. Composite component thickness is a prime determinant of this soft tissue tension. The variable component thickness provided by polyethylene inserts generally allows for 2–3mm incremental change. This study analyzed the effect of 1-mm incremental changes in polyethylene thickness on soft tissue tension. Our hypothesis was that soft tissue tension would be markedly affected by increases in insert thickness. Computer assisted TKA was performed on eight cadaveric knee specimens (four pairs). The knees were passively moved through full flexion-extension range of motion, for each tibial construct thickness. Kinematics were recorded using the computer navigation software. Soft tissue tension was analyzed by measuring compartmental loads. A validated load cell instrumented tibial insert was used to measure medial and lateral compartmental loads independently. The effect of 1-mm increments in polyethylene thickness on compartmental loads was evaluated. An increase in compartmental loads was measured with increasing insert thickness. Loading in contralateral compartments showed differing behaviour, reflecting varying tension in the medial and lateral sides. Many generated loads showed a reduction after reaching a maximal level with further increase in insert thickness (seven of eight specimens), indicative of tissue failure, although there were no overt indications of failure during the procedure. With a 1-mm increase in insert thickness, six of eight specimens showed an increase in peak loads greater than 100N at some point in the testing procedure, although not always with the same shim thickness. Compartmental loads varied as a function of insert thickness. Most specimens showed signs of soft tissue “micro-failure”. The high sensitivity of compartmental loads to a 1-mm incremental increase is significant and has not been previously appreciated, especially intra-operatively. Currently available inserts with 2–3mm incremental sizes may make obtaining optimal soft tissue tension difficult. In addition to the current focus of obtaining accurate leg alignment, further computer-assisted techniques are required to address soft tissue tension.
Optimal soft tissue tension maximises function after total knee arthroplasty (TKA). Excessive tension may lead to stiffness and or pain, while inadequate tension can lead to instability. Composite component thickness is a prime determinant of this soft tissue tension. The thickness provided by polyethylene inserts currently allows for a 2–3 mm incremental change. This study analyses the effect of incremental change in polyethyl-ene thickness on soft tissue tension. Computer assisted (Stryker Knee Nav) TKA was performed on 8 cadaveric knee specimens (4 pairs). Kinematic data was collected through the navigation software. The soft tissue tension was analysed by measuring compartmental loads. A validated load cell instrumented tibial insert was used to measure medial and lateral compartmental loads independently. The effect of 1mm increments in polyethylene thickness on compartmental loads was evaluated. We measured an increase in compartmental loads with increasing insert thickness. The peak loads in each compartment showed different behaviour reflecting varying tension in the medial and lateral sides. The peak loads generated showed a reduction after reaching a maximal level with further increase in insert thickness. With a one mm increase in insert thickness, 75 % of specimens showed greater than 200 % increase in the peak loads in the lateral compartment. Similarly the medial loads showed a greater than 100% increase. Individual specimens showed a high variability in loading patterns. Our study highlights high variation of knee loads present between subjects. The compartmental loads vary as a function of insert thickness. The high sensitivity of compartmental loads with a 1mm increment is significant and has not been previously appreciated, especially intraoperatively. The currently available TKA inserts with 2–3 mm increments may make obtaining optimal soft tissue tension difficult. In addition to the current focus of obtaining accurate leg alignment, further computer aided techniques are required to address soft tissue tension.
Computer assisted navigation (CAN) has been shown to significantly improve the overall alignment obtained after total knee arthroplasty (TKA). Human error and the use of conventional jigs may be the reasons for the inaccuracy of conventional TKA. The impact of computer assisted equipment in surgeon training has not yet been established. Three orthopaedic trainees participated in this prospective study to assess the impact of CAN upon intraoperative alignment. Each trainee’s first five (early group) and last five (late group) TKA’s were included in the study during their three month training period. A total of 30 patients were included in the study. The accuracy of conventional jig positioning was assessed simultaneously using navigation equipment. After this assessment, the actual bony resection was performed using CAN equipment. There was a consistent trend towards improved accuracy between the early and late groups in the majority of parameters assessed. In the early group, the coronal plane tibial alignment was found to be outside the acceptable three degree range in 11 out of 15. In the late group this improved to two out of 15 (p<
0.05). An average of 2.8 degrees of tibial jig deviation during pinning was noted in the early group which improved to one degree in late group. The accuracy of jig placement in both groups was improved by CAN. Computer assisted navigation is helpful in improving the accuracy of trainee surgeons and should prove a useful adjunct in training. Surgical accuracy using conventional jig based systems can be improved with training. Deviation of conventional tibial alignment jig during pinning is a significant factor. This aspect has not been appreciated fully in the past and can be minimised by the use of the navigation equipment. As shown in previous studies, the overall alignment using CAN is superior to what would have been obtained using conventional jigs for TKA.
