Introduction

Undersizing of an uncemented femoral stem is a post-operative description of a radiograph with unclear clinical importance. It may not always translate into a suboptimal clinical outcome.

Recent studies have demonstrated that implant-specific blood metal ion thresholds exist in unilateral and bilateral metal-on-metal (MoM) hip arthroplasty patients, with these thresholds being most effective for identifying patients at low-risk of adverse reactions to metal debris (ARMD). We investigated whether these new blood metal ion thresholds could effectively identify patients at risk of ARMD in an external cohort of MoM hip arthroplasty patients.

We performed a validation study involving 803 MoM hip arthroplasties implanted in 710 patients at three European centres (323=unilateral Birmingham Hip Resurfacing (BHR); 93=bilateral BHR; 294=unilateral Corail-Pinnacle). All patients underwent whole blood metal ion sampling. Patients were divided into those with ARMD (revised for ARMD or ARMD on imaging; n=75), and those without ARMD (n=635). Previously devised implant-specific blood metal ion thresholds (cobalt=2.15μg/l for unilateral BHR; maximum cobalt or chromium=5.5μg/l for bilateral BHR; cobalt=3.57μg/l for unilateral Corail-Pinnacle) were applied to the validation cohort, with receiver operating characteristic curve analysis used to establish the discriminatory characteristics for each respective threshold.

The area under the curve, sensitivity, specificity, positive predictive value and negative predictive value for distinguishing between patients with and without ARMD for each implant-specific threshold were respectively: unilateral BHR=89.4% (95% CI=82.8%-96.0%), 78.9%, 86.7%, 44.1%, 96.9%; bilateral BHR=89.2% (95% CI=81.3%-97.1%), 70.6%, 86.8%, 54.5%, 93.0%; unilateral Corail-Pinnacle=76.9% (95% CI=63.9%-90.0%), 65.0%, 85.4%, 24.5%, 97.1%. The 7μg/l UK MHRA threshold missed significantly more patients with ARMD compared with the implant-specific thresholds (4.9% vs. 2.8%; p=0.0003).

This external multi-centre validation study has confirmed that MoM hip arthroplasty patients with blood metal ion levels below newly devised implant-specific thresholds have a low-risk of ARMD. Compared to implant-specific thresholds, the currently proposed fixed MHRA threshold missed more patients with ARMD.

We recommend using implant-specific thresholds over fixed thresholds when managing MoM hip arthroplasty patients.

When inserting a femoral stem, surgeons make use of many visual and tactile cues to be sure that the implant is correctly sized and well-seated. One such cue is the change of pitch that can be heard when the final femoral broach is inserted. This is known to be important, but has not been widely studied.

We set out to analyse the sounds produced during femoral broaching and implant fixation, and to discover whether the absence of these sounds could predict a poor fixation.

We recorded the sound of femoral broaching and definitive implant insertion, for twenty un-cemented Corail total hip replacements. Procedures were performed by the same surgeon, in the same theatre. The recordings were visualised using audio editing software, and a Fast Fourier Transform was used to identify the dominant audio frequencies.

In 19 of the 20 cases, the final strikes of the final femoral broach displayed a distinctive pattern, with the most prominent frequencies being harmonics (multiples of a fundamental frequency) which had a wavelength directly related to the length of the femoral canal. This contrasts with initial strikes, where multiple unrelated frequencies were present.

Postoperative radiographs were examined by two surgeons independently, to assess implant sizing and positioning. The one case, in which the harmonic pattern was not observed, was found on radiographs to be an undersized, varus malpositioned implant.

We demonstrate that a characteristic frequency pattern is present when impacting cancellous bone with a well-sized and well-placed femoral broach. When the pattern was absent, the broach and implant were undersized and malpositioned. We hypothesise that this pattern arises when broach and femur are vibrating as one, indicating adequate contact with, and compression of, cancellous bone.

Introduction: Computer navigated total knee replacement does not require the use of intramedullary alignment rods, and is thus less invasive than traditional methods.

One previous study has suggested that the computer-assisted technique may reduce blood loss in comparison to traditional methods. This study (Kalairajah et al, 2005) used blood volume loss from drainage bottles as a primary outcome measure (n=60). Hidden (internal) blood losses were not accounted for.

Our study uses a more accurate method of assessing blood loss, and the sample size is larger (n=136; 68 standard TKR versus 68 computer assisted TKR).

Methods: 136 TKR patients were included, of which 68 had standard TKR and 68 computer assisted. Patients were matched such that in each group half had BMI in the range 20–30, and half had BMI between 30–40. Patients were also matched for gender. All patients had Tranexamic acid at the start of the procedure.

Total body blood volume was calculated using the formula of Nadler, Hidalgo & Bloch (1962). This was then used, together with haematocrit and volume re-infused or transfused, to calculate true blood loss, as described by Sehat, Evans, and Newman (2004). This method is considered to be more reliable than measuring drain output, as it takes account of “hidden” losses. The navigated and non-navigated groups were compared using Student’s t-test.

Results: The average blood loss was 583ml in the standard TKR group, and 442ml in the computer assisted TKR group. This difference was statistically significant (p=0.003).

Conclusions: A previous study found reduced blood loss when performing total knee replacement using computer navigation, compared with traditional methods. Our study confirmed this finding, using a larger sample size, and a more reliable method of assessing blood loss.

Our study found that overall blood loss was less for both groups, when compared to the findings of Kalairajah Y et al. We suspect that this difference was due to our departmental policy that all patients receive tranexamic acid at the start of joint replacement procedure.

The accuracy of measurement in computer-assisted total knee arthroplasty is dependent on the quality of data acquisition at the start of the procedure; errors in landmark identification could lead to misalignment and therefore poorer longterm outcomes.

Some navigation systems require the surgeon to explicitly identify the femoral epicondyles in order to calculate the trans-epicondylar axis, whereas other systems are able to interpolate the epicondylar location based on a number of points acquired from the distal femoral surface. Significant inter-observer variability in landmark identification has been previously reported in dry bone studies. The purpose of this study was to test the accuracy of identification of the epicondyles during a simulated total knee replacement on a fresh cadaveric specimen.

An unfixed fresh cadaveric left lower limb was used to perform a navigated total knee replacement using the Orthopilot® (B|Braun-Aesculap, Tuttlingen, Germany) image-free navigation system.

Sixteen surgeons attending an advanced navigation training course were invited to take part. A single consultant surgeon performed initial dissection and pin placement, up to the point of landmark acquisition. Each subject was then asked to use a pointer tool to identify the medial and lateral epicondyles, as they would in an operative situation. Data were recorded by the Orthopilot® system, and exported as a 3D array for further analysis.

Initial visualisation with a 3D scatter plot showed that points were evenly distributed within a circular pattern around each epicondyle. The length of a vector between each point on each epicondyle was calculated in turn. The maximum distances between points were 15.6mm for the medial epicondyle, and 19.9mm for the lateral epicondyle.

We then calculated the length and angulation of the trans-epicondylar axis (TEA) for each observer, equivalent to the vector between each pair of points (medial and lateral epicondyle). An average TEA was calculated, and the range and standard deviation of angulation were determined. In the x axis the range was 16.3° (–8.3° to 7.9°, SD 5.1°), in the y axis the range was 18.7° (–8.7° to 10°, SD 5.2°) and in the z axis the range was 20.5° (–10.1° to 10.4°, SD 6.5°). Range of recorded TEA length was 64.5 to 74.9mm (mean 70.6mm, SD 3.3mm).

We conclude that in this simulated operative scenario, surgeons exhibited considerable variability when locating the epicondyles. Range of angulation of the TEA exceeded 16° (SD > 5.1°) in all 3 planes. We cannot recommend the use of a trans-epicondylar axis determined from 2 single points, as a reliable landmark in navigated total knee replacement.

Computer navigated total knee replacement is less invasive than traditional methods, as it avoids the use of intramedullary alignment rods. A previous study (Kalairajah et al, 2005) has shown that computer-assisted techniques may reduce blood loss in comparison to traditional methods. Our study uses a more accurate method of assessing blood loss, and the sample size is larger.

136 TKR patients were selected from a prospectively collected database of all those undergoing arthroplasty at our institution; 68 had standard TKR and 68 had a computer assisted TKR. In each group, half had BMI in the range 20–30, and half had BMI between 30–40. There were an equal number of males and females in each group. All patients received a standardised anaesthetic, and had tranexamic acid at the start of the procedure.

Total body blood volume was calculated from patient height, weight and sex, using the model described by Nadler, Hidalgo & Bloch (1962). This was then used, together with pre- and post-op haematocrit and volume re-infused or transfused, to calculate true blood loss, as described by Sehat, Evans, and Newman (2004). This method is considered to be more reliable than measuring drain output, as it takes account of “hidden” (internal) losses.

The average blood loss was 603ml in the standard TKR group, and 448ml in the computer assisted TKR group. Student’s t-test showed that this difference was statistically significant (p = 0.007). Regression analysis showed no significant difference between obese and non-obese patients, nor a difference between sexes. Blood loss in both groups was lower than in a previous study, which we attribute to our department’s routine use of tranexamic acid.

We conclude that computer-assisted total knee replacement leads to significant reduction in blood loss when compared with traditional techniques. This confirms previous reports.