Trabecular orientation in the human femur and tibia and the relationship with lower-limb alignment for patients with osteoarthritis of the knee.

Wolff׳s Law suggests that the orientation of trabeculae in human bone changes in response to altered loading patterns. The aim of this study was to investigate trabecular orientation in both the femur and tibia and to compare this with the mechanical axis of the leg. The study involved analysis of radiographs from patients with osteoarthritis of the knee (n=91). For each patient, the trabecular orientation in both the distal femur and proximal tibia was measured from a standard anteroposterior radiograph of the knee and the mechanical axis of the leg was calculated from a long leg view taken while weight bearing. There was a significant correlation between the mechanical axis and the trabecular orientation in each of the regions considered in the femur (r=-0.41, -0.30, 0.52, and 0.23) and tibia (r=-0.27 and 0.31). Multiple regression analysis, with mechanical axis as the dependent variable, produced an R(2) of 0.62. Greater trabecular anisotropy (i.e. greater alignment) was observed in the medial femur and tibia compared to the lateral side (p<0.01). The results give an insight into the trabecular changes that may take place during development of osteoarthritis and following surgery. In particular, we propose that the orientation of the trabeculae in both the distal femur and proximal tibia will reflect the angle of mechanical loading through the knee.

Experience curve compared with manufacturing processes for TKA.

In the experience curve concept set forth by the National Aeronautics and Space Administration (NASA), production time falls by a set percentage every time cumulative production doubles. NASA has established benchmark figures for different manufacturing processes, and we have used these figures in analyzing our first 240 navigated total knee arthroplasties for varus knees. Our experience curve was 93% (P < .001), which is similar to the experience curve (90%) for processes consisting of 25% hand assembly and 75% machining. We suggest that the experience curve may provide a guide for comparing different surgical teams and navigation systems and for resource allocation.

Fluid temperatures during arthroscopic subacromial decompression using a radiofrequency probe.

Our aim was to investigate the temperatures reached in the subacromial space during radiofrequency ablation of the subacromial bursa, in order to see if the probes generate sufficiently high temperatures to cause chondrolysis of the articular surfaces in the glenohumeral joint. We recorded the maximum temperatures in the subacromial space during arthroscopic subacromial decompression on 30 consecutive patients using a sterile digital temperature probe (Series 400, DeRoyal, TN, USA). Both the mean (27.8 degrees C) and maximum (41.8 degrees C) temperatures reached were below the experimental thresholds for chondrocyte damage. At 2 year follow-up we report no cases of chondrolysis or other complications related to the radiofrequency probes and conclude that given the relatively low maximum temperatures generated by the probes the risk of adverse effects is minimal.

The statistical relationship between varus deformity, surgeon's experience, BMI and tourniquet time for computer assisted total knee replacements.

The outcome of total knee arthroplasty (TKA) with severe pre-operative varus deformity is significantly worse than in well aligned knees. Computer navigated TKA has addressed some of the problems by ensuring accurate post-operative alignment. Our aims were to see if navigation could reproducibly correct a varus deformity to 3 degrees of the mechanical axis and to investigate the relationships between the tourniquet time and severity of the preoperative deformity, BMI and a surgeon's experience. The 172 e.motion floating platform TKA's were implanted using the OrthoPilot Navigation system (B Braun-Aesculap, Tuttlingen, Germany). Pre-operative deformity and post-operative alignment were measured by the software. Tourniquet times were recorded automatically. All knees were corrected to within 3 degrees of the neutral axis (mean 0.48 degrees ). Statistically significant relationships between tourniquet time (TT) and degree of pre-operative varus (p<0.001), total number of previous e.motion TKA's performed (p<0.001), and body mass index (p=0.013) were found. A linear relationship between the variables can be expressed as a statistical formula: TT=49.5+PreOp Varus+0.6(BMI)-0.1(total previous). Tourniquet time is measured in minutes and preoperative varus is measured in degrees from the mechanical axis. Total previous is the total number of navigated TKA's implanted by the surgeon. Tourniquet time is increased with larger pre-operative deformities and high BMI and decreased with surgical experience. The formula may give us a method of predicting the length of a procedure for a particular surgical team and may allow us to plan operating lists more accurately.

Factors affecting the learning curve in Computer Assisted Total Knee Arthroplasty.

Mathematical modeling of learning curves have been used in industry since 1936. The advent of Computer Assisted Total Knee Arthroplasty allows the collection of hard data. We present our finding of a statistically significant mathematical model relating operative time, body mass index, experience of the surgeon and preoperative varus deformity in a prospective study of 241 computer assisted total knee arthroplasties. To the best of our knowledge, such a model has not been described before.