The Majority of Total Hip Arthroplasty Patients With a Stiff Spine Do Not Have an Instrumented Fusion.

BACKGROUND: Total hip arthroplasty (THA) patients with limited lumbar flexion (LF) have increased rates of dislocation. An instrumented spinal fusion is a well-recognized cause whose risk increases with increasing number of levels fused. However, many patients without an instrumented fusion (IF) also exhibit abnormal spinopelvic mobility. The purpose of this study was to understand the proportion of THA patients without an IF that have a stiff spine (SS) and behave as if they are surgically fused. METHODS: A retrospective analysis was performed on 6340 primary THA patients, all of whom had preoperative spinopelvic measurements. Any IF of the lumbar spine was observed on the lateral standing radiograph and recorded. SS was classified by LF ≤ 20°, and the percentage of patients with an IF and limited LF was determined. RESULTS: Three hundred fifty-six (6%) patients had a SS, and only 67 (19%) had an IF. Of the entire 6340 patients, 207 (3%) had an IF. Of these 207, only 67 (32%) had a SS. CONCLUSIONS: The vast majority (81%) of THA patients with a SS do not have an IF. We recommend preoperative spinopelvic assessment of all patients undergoing THA, as only a minority of those with limited LF have an IF and may otherwise be overlooked. Lumbar degenerative disc disease is common in THA patients, limits the available LF in the same way an IF might and potentially increases the risk of dislocation in this subset of patients. LEVEL OF EVIDENCE: III.

Variation in femoral anteversion in patients requiring total hip replacement.

BACKGROUND: Optimal implant alignment is important for total hip replacement (THR) longevity. Femoral stem anteversion is influenced by the native femoral anteversion. Knowing a patient's femoral morphology is therefore important when planning optimal THR alignment. We investigated variation in femoral anteversion across a patient population requiring THR. METHODS: Preoperatively, native femoral neck anteversion was measured from 3-dimensional CT reconstructions in 1215 patients. RESULTS: The median femoral anteversion was 14.4° (-27.1-54.5°, IQR 7.4-20.9°). There were significant gender differences (males 12.7°, females 16.0°; p < 0.0001). Femoral anteversion in males decreased significantly with increasing age. 14% of patients had extreme anteversion (<0° or >30°). CONCLUSIONS: This is the largest series investigating native femoral anteversion in a THR population. Patient variation was large and was similar to published findings of a non-THR population. Gender and age-related differences were observed. Native femoral anteversion is patient-specific and should be considered when planning THR.

Two-year serum metal ion levels in minimally invasive total conservative hip resurfacing: preliminary results of a prospective study.

BACKGROUND: Metal-on-metal (MoM) hip resurfacings have been associated with the potential development of metallic debris and its associated pathology. Serum cobalt and chromium levels are a reliable surrogate marker of wear rates in MoM bearings. The aim of the study was to examine the trend in serum metal ion levels in the initial 2-year post-operative period following implantation of the minimally invasive total conservative hip MoM hip resurfacing and to determine whether head size, acetabular component orientation, clinical outcome scores or post-operative range of movement would affect these levels. METHODS: In this prospective cohort study, serum cobalt and chromium ion levels were measured pre- and post-operatively in 25 patients who underwent minimally invasive total conservative hip MoM hip resurfacing. The results were correlated with acetabular component orientation, head size, outcome scores and post-operative range of movement. RESULTS: The mean serum cobalt and chromium levels at 2 years were 1.2 ppb (0.4-4.4 ppb) and 2.1 ppb (0.7-5.7 ppb). The mean cup inclination was 43° (30°-60°) and anteversion was 18° (1°-47°). There was no clear relationship between serum ions and acetabular component orientation, outcome scores or range of movement. Patients with a head size ≤52 mm had significantly higher metal ion levels (cobalt P = 0.02, chromium P = 0.045). CONCLUSION: Our preliminary results show all patients had cobalt and chromium levels below those indicating a high-risk implant, suggesting successful early outcome from minimally invasive total conservative hip resurfacing surgery.

Distribution of chromium and cobalt ions in various blood fractions after resurfacing hip arthroplasty.

The most appropriate blood fraction for the measurement of metal ions in patients with metal-on-metal implants is controversial. We compared chromium (Cr) and cobalt (Co) ion levels in 29 patients after unilateral hip resurfacing with a size 54-mm femoral Birmingham Hip Resurfacing Prosthesis (Smith and Nephew, London, UK). All had well-functioning arthroplasties between 5 and 59 months after implantation. Ion levels were measured in serum, plasma, red cells, and whole blood in each patient. Our results indicate that only very minor amounts of Cr and Co are associated with red blood cells, with most being associated with serum/plasma. Previous studies using corrosion to produce the ion load have showed a predominance of Cr in the red blood cells. They have also shown that the cellular uptake of Cr is an indicator of its valence. This difference in distribution with our results is indirect evidence that the Cr released from wear of this implant is probably in the more benign trivalent form. It also suggests that most of the metal loss from a normally wearing bearing may be from wear rather than corrosion. If blood is to be used to assess rates of wear and systemic ion levels, then serum gives a better reflection of the true levels than red blood cells.