No difference in whole-blood metal ions between 32-mm and 36- to 44-mm femoral heads in metal-on-polyethylene total hip arthroplasty: a 2-year report from a randomised control trial

Aim: To investigate the effect of femoral head size on blood metal-ion levels caused by taper corrosion in metal-on-polyethylene total hip arthroplasty, comparing 36- to 44-mm heads with 32-mm heads. Methods: In a randomised, controlled, single-blinded trial, 96 patients were allocated to receive either a 32-mm metal head or the largest possible metal head (36–44 mm) that could be accommodated in the thinnest available vitamin E, cross-linked polyethylene insert. Blood metal ion levels were collected at 1- and 2-year follow-ups. Results: At 1-year, metal-ion levels did not differ between the groups. The median (interquartile range) blood-ion levels for the 32-mm versus the 36- to 44-mm group were 0.11 µg/L (0.08–0.15) versus 0.12 µg/L (0.08–0.22), p = 0.546, for cobalt, 0.50 µg/L (0.50–0.59) versus 0.50 µg/L (0.50–1.20), p = 0.059, for chromium and 1.58 µg/L (1.38–2.05) versus 1.48 µg/L (1.14–1.87), p = 0.385, for titanium. At 2 years, there was no difference either and the corresponding values were 0.15 µg/L (0.12–0.24) versus 0.18 µg/L (0.12–0.28), p = 0.682 for cobalt, 0.50 µg/L (0.50–0.50) versus 0.50 µg/L (0.50–0.57), p = 0.554, for chromium and 1.54 µg/L (1.16–1.87) versus 1.42 µg/L (1.01–1.72), p = 0.207 for titanium. Conclusions: The use of the largest possible metal head (36–44 mm) compared to a 32-mm head in metal–on-polyethylene bearings does not appear to elevate blood metal-ion levels up to 2 years postoperatively. As taper corrosion is probably time-dependent, longer-term reports are needed to evaluate the association between large metal heads and blood metal ion levels. Trial registration: ClinicalTrials.gov (reg. ID NCT0231 6704)

Early Post-operative Rises in Serum Metal Ion Levels in Total Hip Arthroplasty

Background: Serum Cobalt (Co) and Chromium (Cr) forms part of the diagnostic process for metallosis following Total Hip Arthroplasty (THA). While knowledge exists on longer term metal ion levels, expected early post-operative rises in serum Co and Cr in Metal-on-Polyethylene (MoP) THAs are currently unknown. This study aims to describe early rises in serum Co and Cr at 6 months post-operatively. Methods: A prospective cohort study of 100 consecutive patients with an uncemented titanium stem from a single THA manufacturer was performed. Patients had either a metal (n=50) or ceramic (n=50) head articulating with a highly cross-linked polyethylene. Serum Co and Cr levels were measured 6 months post-operatively. Analysis compared mean values between groups and to determined baseline levels. Subgroup analysis investigated the effect of femoral head size and offset on metal ion levels. Results: A mean difference of 2.259 nmol/L (95% CI 0.449-4.069; p=0.015) was found when comparing 6-month serum Co in the metal head group compared to baseline. No significant differences were found in serum Cr (p=0.943). Mean serum Co levels were higher in the MoP group compared to the CoP (Ceramic-on-Polyethylene) comparator group (p=0.012). There were no differences in serum Cr (p=0.976). Variations in femoral head size and offset did not impact metal ion levels. Conclusion: At six months, a higher magnitude of serum Co exists in metal heads when compared to baseline (p=0.015) and to ceramic heads (p=0.012). Further study is required to determine whether serum concentrations of metal ions will continue to increase over time.

New method to estimate femoral head size from the trochanteric length of the femur

Introduction: Preoperative planning is an important step before any joint replacement surgery. In developing countries standardised radiographs and planning tools might not be available but nevertheless hemiarthroplasties are performed in certain trauma cases. An equation should be devised to allow a preoperative estimation of the expected femoral head size dimensions in those situations. Materials and methods: 35 lower limbs of human cadavers were studied. The estimated femoral head (EFH) size of each femur was obtained by measuring the trochanteric length (TL) (in cm) and using the equation ‘EFH = 16 + (0.7 × TL)’. The hip joint was dissected, and the actual size of the femoral head (AFH) was measured on the specimen. Results: There was a correlation between the EFH and AFH ( p = 0.0001). Accepting a range of ±3 mm the femoral head size was predicted correctly in 31 hips (89%), for ±4 mm in 33 hips (94%) and for ±5 mm in 35 hips (100%), respectively. Conclusions: A simple tape measurement and the equation Femoral head size = {16 + (0.7 × Trochanteric Length)} ±5 mm gives a rather reliable guess for the expected femoral head size. It might be useful as pre-operative planning tool if no standardised radiographs are available.

The effects of venting and capsulotomy on traction force and hip distraction in hip arthroscopy

This study evaluated the effects of venting and capsulotomy on the ratio of normalized distraction distance to traction force, correlating this trend with patient demographic factors. A ratio was chosen to capture the total effect of each intervention on the hip joint. During primary hip arthroscopy, continuous traction force was recorded, and fluoroscopic images were acquired to measure joint distraction before and after the application of traction, venting and interportal capsulotomy. Distraction–traction force ratios were compared using a one-sided paired t-test. A linear regression model was used to determine the relationship between age, sex and body mass index and pre- and post-intervention distraction–traction force ratios. Seventy-two adult patients and 73 hips were included. There was an increase in hip distraction with a decrease in traction force post-venting and capsulotomy (both P’s <0.001). Mean normalized distraction distance increased 1.5% of femoral head size after venting and an additional 2.2% of femoral head size after capsulotomy. Mean traction force decreased 2.2% (14.7 N) after venting and 2.3% (15.3 N) after capsulotomy. Female sex significantly correlated with larger differences in both pre- and post-venting capsulotomy ratios. Venting and capsulotomy both independently improve the ratio of normalized distraction distance to traction force when performed in vivo. However, the effect sizes of each intervention are small and of questionable clinical significance. Specifically, when adequate distraction for safe surgical hip access cannot be obtained despite application of significant traction force, venting and capsulotomy after the application of traction may not afford substantial improvement.

Radiographic Native Femoral Head Size Correlates with Acetabular Cup Size in Primary Total Hip Arthroplasty

There is very little literature on whether a simple radiographic measurement of native femoral head size correlates with implanted acetabular cup size. We hypothesized that there would be a high correlation between the radiographically measured native femoral head and implanted acetabular component size in primary total hip arthroplasty (THA). We conducted a retrospective study of 277 consecutive patients who underwent primary THA. Patients with prior contralateral THA or a history of congenital deformity were excluded (n = 95). We used the postoperative anteroposterior (AP) pelvic view to calibrate the image using known implanted femoral head component size. We then measured the diameter of the contralateral native femoral head under the assumption that femoral heads are generally symmetrical in patients. Two of the authors performed all measurements. To determine if native femoral head size was correlated with acetabular component size we used the Pearson correlation coefficient (r). There was a high correlation between contralateral native femoral head size and acetabular component size (r = 0.86, 95% confidence interval [CI]: 0.82–0.89). The median difference in radiographically measured native femoral head size and acetabular component size was 7 mm (interquartile range [IQR] = 5–8). Our data showed a high correlation between acetabular component size and radiographically measured contralateral native femoral head size, with the difference being 7 mm. In addition to verifying the actual femoral head size with calipers during surgery, this simple radiographic measurement, which could be done on the affected or contralateral hip, may help surgeons to plan better and improve preoperative templating in primary THA.