Introduction: Heterotopic ossification (HO) occurs commonly after total hip arthroplasty (THA). Its severe form can result in impaired range of motion with reduced functional outcome. The rate and severity of HO after hip surface replacement arthroplasty (SRA) have never been well studied.

Methods: Two hundred and ten hips (194 patients) were randomized to receive uncemented metal-on-metal THA (103) or metal-on-metal SRA (107). Standard antero-posterior radiographs of the pelvis were assessed for HO by 2 reviewers at the latest follow-up (minimum of 6 months), using Brooker severity grading and Kjaersgaard-Andersen regional classification.

Results: Pre-operative and post-operative data were similar for both groups. The incidence of HO was 38.5% in the SRA group compared to 32.6% in the THA group (p=0.5). However, there was a significant difference in severity grades for the 2 groups (chi square, p=0.02). According to Brooker_s classification, nearly half of HO was of grade 2 in SRA and of grade 1 in THA. SRA was associated with significantly higher rates of severe HO (grades 3 and 4) than THA (12.5% vs. 2.2%; p=0.009). Inter-rater agreement for Brooker grading was excellent (Cohen_s kappa, 0.88; p< 0.01). HO in SRA involved both the central and lateral regions in 26% of cases, whereas only 3% of HO in THA showed such a pattern (p=0.025). Risk factors, such as male gender, osteoarthritis, bilateral predilection, and previous history of HO, were observed in both groups. Patients with HO had reduced internal hip rotation (16.4° vs. 22.2°; p=0.02) and a higher incidence of postoperative hip pain (52% vs. 30%; p=0.04), but comparable functional outcome scores.

Discussion: The incidence of HO after hip arthroplasty seems to be determined by patient-related factors. However, HO severity appears to be associated with local surgical factors and thereby SRA may result in more severe HO than THA. An extensive surgical approach, additional soft tissue release and the blunt damage occurring in gluteal muscles with SRA may signal the induction of more severe HO. Peri-operative deposition of bone debris derived from femoral head preparation may also play a role by transplanting osteoprogenitor cells. Surgeons must be aware of this risk of severe HO when offering SRA as an alternative treatment to younger patients. Routine prophylaxis with NSAIDs needs to be considered in these patients. A meticulous surgical technique to reduce muscle damage, pulsed lavage to clear bone debris, and debridement of necrotic tissue, may help to decrease the risk of severe HO in SRA.

Background and purpose of the study: the anterior cruciate ligament (ACL) is a continuum of fibres which are differently recruited through range of motion. Two main functional bundles can be identified: the postero-lateral bundle (PLB) which is taut exclusively towards extension and the anteromedial bundle (AMB) which is taut through full range of motion. The purpose of this investigation was to assess the relative contribution of the bundles to intact knee kinematics.

Material and methods: fourteen intact cadaver knees were instrumented in a non-ferromagnetic rig and six degrees of freedom kinematics through flexion-extension was recorded with an electromagnetic device under the application of a 90N anterior force or a 5Nm internal rotation torque. The AMB and PLB were alternatively cut first in each knee and knee kinematics was recorded. The other bundle was then dissected and ACL deficient knee kinematics tested.

Results: when the AMB was cut anterior tibial translation increased and no effects on rotations were recorded. When the PLB was first cut no significant effects on anterior laxity were observed. Different rotational responses were observed in different knees. After the section of both bundles a larger increase in anterior laxity was observed. The changes in rotation differed from knee to knee.

Discussion: The AMB is a primary restraint against anterior tibial translation and has a small and variable effect on rotations. The PLB is a secondary restraint against anterior tibial translation in extension and maintains normal rotational laxity in AMB deficient knees. Therefore, reconstruction of both bundles is theoretically advantageous to restore both intact knee anteroposte-rior and rotational laxity.