The routine use of a fixed distal femoral resection angle in total knee arthroplasty (TKA) assumes little or no variation in the angle between the anatomical and mechanical femoral axes (FMA angle) in different patients. The aims of this study were threefold, firstly to investigate the distribution of FMA angle in TKA patients, secondly to identify any correlation between the FMA angle and the pre-operative coronal mechanical femoro-tibial (MFT) angle and in addition to assess post-operative MFT angle with fixed or variable distal femoral resection angles.

277 primary TKAs were performed using either fixed or variable distal femoral resection angles (174 and 103 TKAs respectively), with intramedullary femoral and extramedullary tibial jigs. The variable distal femoral resection angles were equal to the FMA angle measured on pre-operative Hip-Knee-Ankle (HKA) digital radiographs for each patient. Outcomes were assessed by measuring the FMA angle and the pre- and post-operative MFT angles on HKA radiographs.

The FMA angle ranged from 2° to 9° (mean 5.9°). Both cohorts showed a correlation between FMA and pre-operative MFT angles (fixed: r = -0.499, variable: r = -0.346) with valgus knees having lower FMA angles. Post-operative coronal alignment within ±5° increased from 86% in the fixed angle group to 96% when using a variable angle, p = 0.025. For post-operative limb alignment within ±3°, accuracy improved from 67% (fixed) to 85% (variable), p = 0.002.

These results show that the use of a fixed distal femoral resection angle is a source of error regarding post-operative coronal limb malalignment. The correlation between the FMA angle and pre-operative varus-valgus alignment supports the rational of recommending the adjustment of the resection angle according to the pre-operative deformity (3°-5° for valgus, 6°-8° for varus) in cases where HKA radiographs are not available for pre-operative planning.

Many studies have already been published to prove the improved accuracy in achieving the ideal post-operative long leg alignment when using computer navigation in total knee arthroplasty (TKA). Surgeons who use traditional instrumentation with a fixed distal femoral resection angle (most commonly 6°) assume little or no variation in the angle between the anatomical and mechanical axis of the femur (FMA angle) in different patients.

The aims of this study were to investigate the distribution of the FMA angle in pathological knees of patients about to undergo TKA and to analyse if there was any correlation between the FMA angle and the pre-operative lower limb alignment in the coronal plane (varus or valgus).

The study consisted of 158 consecutive patients undergoing 174 primary TKA between January and October 2007. All patients had pre-operative digital Hip-Knee-Ankle radiographs. The FMA angle and the mechanical femorotibial angle (MFT angle) were measured in all cases. Intra- and inter-observer variation was measured by second observer readings and repeated measurements.

The mean age of the study cohort was 69.9 years (SD 8.7 years). There were 75 male and 99 female knees. The repeatability for measurement of the FMA angle was good (intra-observer Intra Correlation Coefficient (ICC) = 0.91, inter-observer ICC = 0.85) and for the measurement of MFT angle was very good (intra-observer ICC = 0.99, inter-observer ICC = 0.99). There were 135 knees with a varus or neutral alignment and 39 knees with valgus alignment. The median alignment was 6.5° varus ranging from 23° varus to 16° valgus. The FMA angle was between 2° and 9°, with a median of 6°. The FMA angle was 6° in 35.4% of cases, 5° in 22.9% and 7° in 18.3%. There was a statistical significant correlation between the FMA angle and the pre-operative lower limb alignment (Pearson correlation coefficient = −0.5, p < 0.001), with valgus knees having on average a lower FMA angle. The group of females and males had statistically different FMA angles (Mann-Whitney, p < 0.001) with females having on average a lower FMA angle. Cluster analysis based on the original clinical definitions of severe varus, varus and valgus gave three groups of FMA angle for MFT angle < 8° varus, MFT angle of 8° varus to 1° valgus and MFT angle > 1° valgus. There was a statistically significant difference in median FMA angle between these three groups (Kruskal-Wallis, p < 0.001).

This study indicates that one of the main reasons why optimal post-operative coronal alignment cannot be achieved with a fixed distal femoral resection angle is the fact that the FMA angle has a wide, natural distribution. It is possible that better results may be achieved with traditional instrumentation by individual measurement of FMA angle for each patient pre-operatively and adjusting the distal femoral resection to account for this. However, with computer navigation the distal femoral cut is adjusted for each patient.

The recent introduction of intra-articular local anaesthetic techniques following total joint arthroplasty have demonstrated improvements in post-operative pain control, early joint mobilisation and may contribute to early discharge. Following intra-operative infiltration, the CALEDonian Technique requires ropivicaine to be administered postoperatively via an epidural catheter and filter, 20 to 40 ml over 60 seconds, on three occasions. Epidural filter catheters are used to prevent bacterial contamination during injection and have demonstrated 100% efficacy when intact. However, on internal departmental audit we have become aware of a filter membrane rupture rate approximating 10%. We therefore investigated the variables of syringe size and rate of administration in the occurrence of filter ruptures.

Using a standard primed epidural catheter set (Perifix B Braun), pressure measurements were taken pre- and post-filter. Recordings using the filter with attached catheter tubing were undertaken using 5 ml, 10 ml and 20 ml syringes (n=10) during infusion of a standard 5 ml volume. Controlled (over 1 min and following departmental protocol) and forced (over less than 15 seconds) infusions were undertaken manually and the experiment also undertaken using an automated syringe driver with 40 ml infused at a rate of 400 ml/hr. Each experiment was repeated ten times. Infusion pressures were measured and filters examined for evidence of rupture.

Using departmental protocol, controlled infusions independent of syringe size generated consistently low pressures, averaging 115 kPa, with no filter ruptures. Forced administration, independent of syringe size, generated pressures averaging 625.1 kPa. This is above the filter threshold and resulted in almost universal filter rupture. An automated device infusing at a rate of 400 ml/hr again generated low pressures and no filter ruptures.

Our study demonstrates low infusion pressures and no filter ruptures, independent of syringe size, when departmental protocol is adhered to suggesting that a human element may be in-part responsible for filter rupture. Although the technique currently used is safe, our audit has raised awareness of a potential difficulty and has lead to re-education of staff involved in this process. Consideration is being given to the possibility of automating infusions in the future and a process of re-audit of filter rupture will be undertaken.

Computer assisted total knee arthroplasty (TKA) enables the measurement of the dynamics of the knee both before and after the implant of the prosthesis. Much time has been spent looking at the outcomes of navigated TKA however less time has been invested on understanding how the data collected pre-operatively can inform the surgeon and help the surgical decision making process. The aim of this work was to use navigation as a tool to quantify and classify preoperatively valgus knees.

Between August 2006 and September 2007 a group of 51 patients who demonstrated intra-operative initial neutral or valgus aligned knees underwent navigated TKA using the Columbus knee prosthesis and the Orthopilot^® navigation system (BBraun, Tuttlingen, Germany). Demographic data were recorded, along with the preoperative radiograph appearance and clinical assessment of alignment. During the surgery the approach used and the knee mechanical femorotibial (MFT) angle though the range of flexion were recorded. The knees were then categorised as either “True” valgus or “False” valgus based on whether the MFT angle at 30°, 60° and 90° flexion was still valgus (True) or had gone into varus (False).

Five patients were excluded from the study group as they had incomplete data in knee flexion. Of the remaining 46 patients, 28 were True valgus and 18 were False valgus. For the two groups demographic data were compared. Male to female ratio was 9:19 for the True valgus and 4:14 for the False valgus. The mean age of the True group was 70 years (range 52–85 years) and the False was 69 years (range 53–84 years). For BMI the True group had mean of 31 (range 20–40) and False of 33 (range 26–42). Twenty-five of the 28 True valgus knees showed preoperative evidence of clinical genu valgum deformity and radiologic evidence of predominantly lateral compartment osteoarthritis. Five patients had ipsilateral hip replacements in the past and five had rheumatoid arthritis. Seventeen were operated by lateral parapatellar approach. Eighteen required ilio-tibial band release with additional lateral collateral ligament release in five knees. Six true valgus knees did not require any soft tissue release. Five patients required lateral retinacular release to achieve thumb free patellar tracking. The median operating time for the True valgus group was 80 mins. Ten of the 18 false valgus knees showed evidence of clinical varus deformity and radiological evidence of predominantly medial compartment osteoarthritis. Only one patient had an ipsilateral hip replacement in the past and one had rheumatoid arthritis. All 18 knees underwent TKA by medial parapatellar approach, requiring no additional soft tissue release in 17 knees and a moderate release in one knee. The median operating time for the False valgus group was 60 mins.

True valgus knees had more significant deformities clinically and radiologically, longer surgical time and more incidence of soft tissue release when compared to the False valgus knees. False valgus knees behaved like varus knees clinically, radiologically and intra-operatively and should therefore be treated as such when making surgical choices.