Removal of bullets retained within joints is indicated to prevent mechanical blockade, 3rd body wear and resultant arthritis, plus lead arthropathy and systemic lead poisoning. The literature is sparse on this subject, with mostly sporadic case reports utilizing hip arthroscopy. We report on the largest series of removal of bullets from the hip joints using open surgical. We reviewed prospectively collected data of patients who presented to a single institution with civilian gunshot injuries that breached the hip joint between 01 January 2009 and 31 December 2022. We included all cases where the bullet was retained within the hip joint area. Exclusion criteria: cases where the hip joint was not breached, bullets were not retained around the hip area or cases with isolated acetabulum involvement. One hundred and eighteen (118) patients were identified. One patient was excluded as the bullet embedded in the femur neck was sustained 10 years earlier. Of the remaining 117 patients, 70 had retained bullets around the hip joint. In 44 patients we undertook bullet removal using the followingsurgical hip dislocation (n = 18), hip arthrotomy (n = 18), removal at site of fracture fixation/replacement (n = 2), posterior wall osteotomy (n = 1), direct removal without capsulotomy (tractotomy) (n = 5). In 26 patients we did not remove bullets for the following reasons: final location was extra-capsular embedded in the soft tissues (n=17), clinical decision to not remove (n=4), patients’ clinical condition did not allow for further surgery (n= 4) and patient refusal (n=1). No patients underwent hip arthroscopy. With adequate pre-operative imaging and surgical planning, safe surgical removal of retained bullets in the hip joint can be achieved without the use of hip arthroscopy; using the traditional open surgical approaches of arthrotomy, tractotomy and surgical hip dislocation

Aims. Developmental dysplasia of the hip (DDH) is a complex musculoskeletal disease that occurs mostly in children. This study aimed to investigate the molecular changes in the hip joint capsule of patients with DDH. Methods. High-throughput sequencing was used to identify genes that were differentially expressed in hip joint capsules between healthy controls and DDH patients. Biological assays including cell cycle, viability, apoptosis, immunofluorescence, reverse transcription polymerase chain reaction (RT-PCR), and western blotting were performed to determine the roles of the differentially expressed genes in DDH pathology. Results. More than 1,000 genes were differentially expressed in hip joint capsules between healthy controls and DDH. Both gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that extracellular matrix (ECM) modifications, muscle system processes, and cell proliferation were markedly influenced by the differentially expressed genes. Expression of Collagen Type I Alpha 1 Chain (COL1A1), COL3A1, matrix metalloproteinase-1 (MMP1), MMP3, MMP9, and MMP13 was downregulated in DDH, with the loss of collagen fibres in the joint capsule. Expression of transforming growth factor beta 1 (TGF-β1) was downregulated, while that of TGF-β2, Mothers against decapentaplegic homolog 3 (SMAD3), and WNT11 were upregulated in DDH, and alpha smooth muscle actin (αSMA), a key myofibroblast marker, showed marginal increase. In vitro studies showed that fibroblast proliferation was suppressed in DDH, which was associated with cell cycle arrest in G0/G1 and G2/M phases. Cell cycle regulators including Cyclin B1 (CCNB1), Cyclin E2 (CCNE2), Cyclin A2 (CCNA2), Cyclin-dependent kinase 1 (CDK1), E2F1, cell division cycle 6 (CDC6), and CDC7 were downregulated in DDH. Conclusion. DDH is associated with the loss of collagen fibres and fibroblasts, which may cause loose joint capsule formation. However, the degree of differentiation of fibroblasts to myofibroblasts needs further study. Cite this article: Bone Joint Res 2021;10(9):558–570

Abstract. OBJECTIVES. Abnormal joint mechanics have been proposed as adversely affecting natural hip joint tribology, whereby increased stress on the articular cartilage from abnormal loading leads to joint degeneration. The aim of this project was to assess the damage caused by different loading conditions on the articular surfaces of the porcine hip joint in an experimental simulator. METHODS. Porcine hip joints were dissected and mounted in a single station hip simulator (SimSol, UK) and tested under loading scenarios (that corresponded to equivalent of different body mass index's’ (BMI) in humans), as follows:“Normal” (n=4), the loading cycle consisted of a simplified gait cycle based on a scaled version of a simplified twin-peak human gait cycle, the peak load was 900N (representative of a healthy BMI). Representative of an “Overweight” BMI (n=3), as the normal cycle with a peak load of 1,130N Representative of an “Obese” BMI (n=1), as the normal cycle with a peak load of 1,340N Tests were conducted at 1Hz for 14,400 cycles in Ringers solution; photogrammetry was used to characterise the appearance of the cartilage and labrum pre, during and post simulation. the appearance and location of damage was recorded. RESULTS. No significant damage was observed for samples tested under normal conditions. Following “overweight” condition testing, tears and detachment of the labrum were observed during testing in two (of three) samples. In addition to damaged observed in “overweight” tested samples the “obese” showed similar damage and also cartilage bruising and wear tracks on the articular surface of the acetabulum. DISCUSSION. The absence of damage in “normal” loading provides evidence that this is an appropriate methodology and loading regime for porcine hips. Increased damage with increasing loads demonstrates the potential to develop further this experimental simulation to assess adverse loading in natural hip joints. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

The interleukin-6/gp130-associated Janus Kinases/STAT3 axis is known to play an important role in mediating inflammatory signals, resulting in production of matrix metalloproteinase-3 (MMP-3). The hip joints with rapidly destructive coxopathy (RDC) demonstrate rapid chondrolysis, probably by increased production of MMP-3 observed in the early stage of RDC. In the recent study, no apparent activation of STAT3 has been shown in the synovial tissues obtained from the osteoarthritic joint at operation. However, no data are currently available on STAT3 activation in the synovial tissues in the early stage of RDC. This study aimed to elucidate STAT3 activation in the synovial tissues in the early stage of RDC. Synovial tissues within 7 months from the disease onset were obtained from four RDC patients with femoral head destruction and high serum levels of MMP-3. RDC synovial tissues showed the synovial lining hyperplasia with an increase of CD68-positive macrophages and CD3-positive T lymphocytes. STAT3 phosphorylation was found in the synovial tissues by immunohistochemistry using anti-phospho-STAT3 antibody. The majority of phospho-STAT3-positive cells were the synovial lining cells and exhibited negative expression of macrophage or T cell marker. Treatment with tofacitinib, a Janus Kinase inhibitor, resulted in a decrease in phospho-STAT3-positive cells, especially with high intensity, indicating effective suppression of STAT3 activation in RDC synovial tissues. Inhibitory effect of tofacitinib could act through the Janus Kinase/STAT3 axis in the synovial tissues in the early stage of RDC. Therefore, STAT3 may be a potential therapeutic target for prevention of joint structural damage in RDC. Acknowledgements: This study was supported by Katakami Foundation for Clinical Research

Aims. To describe the epidemiology of acetabular fractures including patient characteristics, injury mechanisms, fracture patterns, treatment, and mortality. Methods. We retrieved information from the Swedish Fracture Register (SFR) on all patients with acetabular fractures, of the native hip joint in the adult skeleton, sustained between 2014 and 2020. Study variables included patient age, sex, injury date, injury mechanism, fracture classification, treatment, and mortality. Results. In total, 2,132 patients with acetabular fractures from the SFR were included in the study. The majority of the patients were male (62%) and aged over 70 years old (62%). For patients aged > 70 years, the 30-day mortality was 8% and one-year mortality 24%. For patients aged ≤ 70 years, the 30-day mortality was 0.2% and one-year mortality 2%. Low-energy injuries (63%) and anterior wall fractures (20%) were most common. Treatment was most often non-surgical (75%). Conclusion. The majority of patients who sustain an acetabular fracture are elderly (> 70 years), of male sex, and the fracture most commonly occurs after a simple, low-energy fall. Non-surgical treatment is chosen in the majority of acetabular fracture patients. The one-year mortality for elderly patients with acetabular fracture is similar to the mortality after hip fracture, and a similar multidisciplinary approach to care for these patients should be considered. Cite this article: Bone Jt Open 2023;4(9):652–658

Aims. The aim of this study was to investigate the association between fracture displacement and survivorship of the native hip joint without conversion to a total hip arthroplasty (THA), and to determine predictors for conversion to THA in patients treated nonoperatively for acetabular fractures. Methods. A multicentre cross-sectional study was performed in 170 patients who were treated nonoperatively for an acetabular fracture in three level 1 trauma centres. Using the post-injury diagnostic CT scan, the maximum gap and step-off values in the weightbearing dome were digitally measured by two trauma surgeons. Native hip survival was reported using Kaplan-Meier curves. Predictors for conversion to THA were determined using Cox regression analysis. Results. Of 170 patients, 22 (13%) subsequently received a THA. Native hip survival in patients with a step-off ≤ 2 mm, > 2 to 4 mm, or > 4 mm differed at five-year follow-up (respectively: 94% vs 70% vs 74%). Native hip survival in patients with a gap ≤ 2 mm, > 2 to 4 mm, or > 4 mm differed at five-year follow-up (respectively: 100% vs 84% vs 78%). Step-off displacement > 2 mm (> 2 to 4 mm hazard ratio (HR) 4.9, > 4 mm HR 5.6) and age > 60 years (HR 2.9) were independent predictors for conversion to THA at follow-up. Conclusion. Patients with minimally displaced acetabular fractures who opt for nonoperative fracture treatment may be informed that fracture displacement (e.g. gap and step-off) up to 2 mm, as measured on CT images, results in limited risk on conversion to THA. Step-off ≥ 2 mm and age > 60 years are predictors for conversion to THA and can be helpful in the shared decision-making process. Cite this article: Bone Joint J 2023;105-B(9):1020–1029

Despite improvements in techniques and materials, aseptic loosening of artificial hip joints remains as the most serious problem. This study investigated mechanical and biological effects of biocompatible 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer on prevention of aseptic loosening. To examine mechanical effects of MPC grafting, we performed hip simulator tests (3million cycles) using cross-linked polyethylene (CL-PE) liners with or without nano-grafting of MPC onto articulating surface (MPC liner/CL-PE liner) and PE liner against CoCrMo heads. To examine biological responses of macrophages and osteoblasts, we prepared MPC nanoparticles (500nm). Using in vitro/vivo murine particle-induced osteolysis model, we examined biological effects of MPC nanoparticles on osteoclastogenesis. The friction torque was about 90% lower in MPC liners than control liners. Total amounts of wear produced from MPC liner was about 1/5and 1/30 of those from CL-PE and PE liners, respectively. Three-dimensional analysis and SEM analysis of MPC liners revealed no or little wear. The effect of MPC nanografting was maintained even after the test, because XPS analysis confirmed the remainder of specific spectra of MPC on the liner surface. When nanoparticles were exposed to cultured mouse macrophages, MPC nano particles were hardly phagocytosed by macrophages and did not enhance the concentration of bone resorptive cytokines and PGE2. Furthermore, culture medium of macrophages exposed to MPC nanoparticles did not induce RANKL expression in osteoblasts and osteoclastogenesis from bone marrow cells. In vivo murine osteolysis model, particle-induced bone resorption was hardly observed in mice implanted MPC nanoparticles. Some medical devices grafted MPC onto itssurface have been already used under authorization of the FDA. This study demonstrated that MPC grafting markedly decreases wear production. In addition, even if wear particles are produced, they are biologically inert in respect to phagocytosis by macrophages and subsequent resorptive actions, suggesting an epochal improvement of artificial hip joints preventing aseptic loosening

Aims. Dislocation of the hip remains a major complication after periacetabular tumour resection and endoprosthetic reconstruction. The position of the acetabular component is an important modifiable factor for surgeons in determining the risk of postoperative dislocation. We investigated the significance of horizontal, vertical, and sagittal displacement of the hip centre of rotation (COR) on postoperative dislocation using a CT-based 3D model, as well as other potential risk factors for dislocation. Methods. A total of 122 patients who underwent reconstruction following resection of periacetabular tumour between January 2011 and January 2020 were studied. The risk factors for dislocation were investigated with univariate and multivariate logistic regression analysis on patient-specific, resection-specific, and reconstruction-specific variables. Results. The dislocation rate was 13.9% (n = 17). The hip COR was found to be significantly shifted anteriorly and inferiorly in most patients in the dislocation group compared with the non-dislocation group. Three independent risk factors were found to be related to dislocation: resection of gluteus medius (odds ratio (OR) 3.68 (95% confidence interval (CI) 1.24 to 19.70); p = 0.039), vertical shift of COR > 18 mm (OR 24.8 (95% CI 6.23 to 128.00); p = 0.001), and sagittal shift of COR > 20 mm (OR 6.22 (95% CI 1.33 to 32.2); p = 0.026). Conclusion. Among the 17 patients who dislocated, 70.3% (n = 12) were anterior dislocations. Three independent risk factors were identified, suggesting the importance of proper restoration of the COR and the role of the gluteus medius in maintaining hip joint stability. Cite this article: Bone Joint J 2022;104-B(10):1180–1188

Aims. The aim of this study was to describe services available to patients with periprosthetic femoral fracture (PPFF) in England and Wales, with focus on variation between centres and areas for care improvement. Methods. This work used data freely available from the National Hip Fracture Database (NHFD) facilities survey in 2021, which asked 21 questions about the care of patients with PPFFs, and nine relating to clinical decision-making around a hypothetical case. Results. Of 174 centres contributing data to the NHFD, 161 provided full responses and 139 submitted data on PPFF. Lack of resources was cited as the main reason for not submitting data. Surgeon (44.6%) and theatre (29.7%) availability were reported as the primary reasons for surgical delay beyond 36 hours. Less than half had a formal process for a specialist surgeon to operate on PPFF at least every other day. The median number of specialist surgeons at each centre was four (interquartile range (IQR) 3 to 6) for PPFF around both hips and knees. Around one-third of centres reported having one dedicated theatre list per week. The routine discussion of patients with PPFF at local and regional multidisciplinary team meetings was lower than that for all-cause revision arthroplasties. Six centres reported transferring all patients with PPFF around a hip joint to another centre for surgery, and this was an occasional practice for a further 34. The management of the hypothetical clinical scenario was varied, with 75 centres proposing ORIF, 35 suggested revision surgery and 48 proposed a combination of both revision and fixation. Conclusion. There is considerable variation in both the organization of PPFF services England and Wales, and in the approach taken to an individual case. The rising incidence of PPFF and complexity of these patients highlight the need for pathway development. The adoption of networks may reduce variability and improve outcomes for patients with PPFF. Cite this article: Bone Jt Open 2023;4(5):378–384

Abstract. Objectives. Hip joint laxity after total hip arthroplasty (THA) has been considered to cause microseparation and lead to complications, including wear and dislocation. In the native hip, the hip capsular ligaments may tighten at the limits of range of hip motion and provide a passive stabilising force preventing edge loading and reduce the risk of dislocation. Previous attempts to characterise mechanical properties of hip capsular ligaments have been largely variable and there are no cadaveric studies quantifying the force contributions of each ligament in different hip positions. In this study we quantify the passive force contribution of the hip capsular ligaments throughout a complete range of motion (ROM). Methods. Nine human cadaveric hip specimens (6 males and 3 females) with mean age of (76.4 ± 9.0 years) were skeletonised, preserving the capsular ligaments. Prepared specimens were tested in a 6 degree of freedom system to assess ROM with 5 Nm torque applied in external and internal rotation throughout hip flexion and extension. Capsular ligaments were resected in a stepwise fashion to assess internal force contributions of the iliofemoral (superior and inferior), pubofemoral, and ischiofemoral ligaments during ROM. Results. In external rotation, the superior and inferior iliofemoral ligament minimum force contributions were (136.52 ± 27.15 N) in flexion and (82.40 ± 27.85 N) in extension, respectively. In internal rotation, the ischiofemoral ligament force contributions were dominant in adducted-flexion positions and abducted-extension positions. Conclusions. These findings provide insights into the primary capsular structures that stabilise the hip joint in different manoeuvres. This data allows for an improved understanding of which capsular ligaments contribute the most to hip stability and has important implications for choosing surgical approaches and repair strategies to minimise complications related to joint instability. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Objectives. To validate the precision of digitally reconstructed radiograph (DRR) radiostereometric analysis (RSA) and the model-based method (MBM) RSA with respect to benchmark marker-based (MM) RSA for evaluation of kinematics in the native hip joint. Methods. Seven human cadaveric hemipelves were CT scanned and bone models were segmented. Tantalum beads were placed in the pelvis and proximal femoral bone. RSA recordings of the hips were performed during flexion, adduction and internal rotation. Stereoradiographic recordings were all analyzed with DRR, MBM and MM. Migration results for the MBM and DRR with respect to MM were compared. Precision was assessed as systematic bias (mean difference) and random variation (Pitman’s test for equal variance). Results. A total of 288 dynamic RSA images were analyzed. Systematic bias for DRR and MBM with respect to MM in translations (p < 0.018 mm) and rotations (p < 0.009°) were approximately 0. Pitman’s test showed lower random variation in all degrees of freedom for DRR compared with MBM (p < 0.001). Conclusion. Systematic error was approximately 0 for both DRR or MBM. However, precision of DRR was statistically significantly better than MBM. Since DRR does not require marker insertion it can be used for investigation of preoperative hip kinematics in comparison with the postoperative results after joint preserving hip surgery. . Cite this article: L. Hansen, S. De Raedt, P. B. Jørgensen, B. Mygind-Klavsen, B. Kaptein, M. Stilling. Marker free model-based radiostereometric analysis for evaluation of hip joint kinematics: A validation study. Bone Joint Res 2018;7:379–387. DOI: 10.1302/2046-3758.76.BJR-2017-0268.R1

Hip osteoarthritis (OA) is a disorder of high socio-economic relevance. The causes of hip osteoarthritis are multifactorial; however, the epidemiological literature regularly cites occupational tasks, such as heavy lifting and carrying, as a risk factor for the development of hip OA. The level of mechanical stresses upon the hip joint caused by occupational tasks remain largely unclear, however. This project sought to quantify the levels of stresses upon the hip joint during occupational tasks. In particular we were interested in comparing load as well as stress levels from everyday activities with occupational tasks typically performed by blue collar workers. Sectors and occupational activities presenting a high potential for stress upon the hip joint were identified by means of a survey conducted among accident insurance institutions. Lifting, carrying and load transfer (25 to 50 kg), ladder climbing and stair climbing (without additional load and with an additional load of 25 kg) were selected from among these sectors and activities for the purpose of the study. Laboratory measurements were performed in which motion capturing and a range of force measurement apparatus were used to record and evaluate the performance of the selected tasks by 12 skilled workers from a number of sectors. multi-body simulation was used to calculate the stress in the form of hip-joint contact forces. The contact pressures and their geometric distribution on the cartilage surfaces of the hip joint were then calculated from these results by means of finite-element analysis. This produced an indicator for the strain upon the hip joint. The highest hip-joint forces, at (637±148)% of the body weight, occurred during handling of the 50 kg load. This corresponded to 1.7 times the stress arising during walking, at (368±78)% of the body weight. Significantly higher hip-joint forces compared to those arising during walking were observed for the carrying of loads of 40 kg and 50 kg, the handling of loads of 25 kg, 40 kg and 50 kg, and stair climbing with an additional load of 25 kg. Maximum contact pressures of 24.1 MPa were computed during the finite-element analysis (lifting of 50 kg); only very small regions of the joint surface were however affected by these high pressures. During walking, the maximum pressure reached 15 MPa. The results obtained provide a quantitative overview of the strains upon the hip joint during occupational and everyday tasks. They constitute an aid to future quantitative exposure assessments in a range of sectors and occupational fields, and thus contribute to improving estimation of the relevance of stresses of occupational origin to the incidence of hip OA

Aims. In the native hip, the hip capsular ligaments tighten at the limits of range of hip motion and may provide a passive stabilizing force to protect the hip against edge loading. In this study we quantified the stabilizing force vectors generated by capsular ligaments at extreme range of motion (ROM), and examined their ability to prevent edge loading. Methods. Torque-rotation curves were obtained from nine cadaveric hips to define the rotational restraint contributions of the capsular ligaments in 36 positions. A ligament model was developed to determine the line-of-action and effective moment arms of the medial/lateral iliofemoral, ischiofemoral, and pubofemoral ligaments in all positions. The functioning ligament forces and stiffness were determined at 5 Nm rotational restraint. In each position, the contribution of engaged capsular ligaments to the joint reaction force was used to evaluate the net force vector generated by the capsule. Results. The medial and lateral arms of the iliofemoral ligament generated the highest inbound force vector in positions combining extension and adduction providing anterior stability. The ischiofemoral ligament generated the highest inbound force in flexion with adduction and internal rotation (FADIR), reducing the risk of posterior dislocation. In this position the hip joint reaction force moved 0.8° inbound per Nm of internal capsular restraint, preventing edge loading. Conclusion. The capsular ligaments contribute to keep the joint force vector inbound from the edge of the acetabulum at extreme ROM. Preservation and appropriate tensioning of these structures following any type of hip surgery may be crucial to minimizing complications related to joint instability. Cite this article: Bone Joint Res 2021;10(9):594–601

Introduction. Soft tissue artefact (STA) affects the kinematics retrieved with skin marker-based motion capture, and thus influences the outcomes of biomechanical models that rely on such kinematics. To date, compensation for STA remains an unsolved challenge due to its complexity. Factors include its dependency on subject, on motion activity and on skin-marker configuration, its non-linearity over the movement cycle, and the scarcity of reference in-vivo estimations. The objective of this study was extending the existing knowledge of the effects of STA on the kinematics of the hip joint and on the hip joint center location, by quantifying them for a sample total hip arthroplasty (THA) population, for a broader range of activities of daily living (ADLs). Methods. Four activities of daily living (overground gait, stairs descent, chair rise and putting on socks) were measured simultaneously with optical motion capture (MC) at 100 Hz and with a movable single-plane video-fluoroscopy system (VF) at 25 Hz, for fifteen patients with successful total hip arthroplasty (THA). The joint segment positions were computed by least-square fitting for MC and by semi-automatic 2D/3D registration for VF. Anatomical coordinate systems were defined for each joint segment based on skin markers location at a reference standing position. Errors induced by STA on the retrieved joint motion were computed as the difference between MC-based kinematics and the reference VF-based kinematics. Statistical analysis was carried out to determine the whether the differences between the kinematics obtained with the two methods were significant. Results. MC underestimated the ROM of the hip joint for all activities. The ROM for the flexion-extension was underestimated by on average 4.1°, 6.5°, 8.0° and 6.9° for gait, stair decent, chair rise and putting on socks respectively. Overall, during dynamic activities the hip joint was less flexed, more adducted and more internally rotated as retrieved using MC, compared to VF. The flexion angle was underestimated by MC during late stance phase and early swing phase for both gait and stairs descent. The internal rotation of the hip was overestimated by MC throughout the whole cycle of each activity. MC error for the thigh was larger than the MC error for the pelvis. MC errors above 1 cm were observed for the location of the hip joint center, with higher values for the cranial-caudal direction. Discussion. Reduced ROM supports the notion that skin-sliding is a major contributor to STA. The underestimation of hip flexion appeared to be positively correlated to the degree of flexion. Larger skin deformation and sliding occurring for the thigh at higher hip flexion angles may produce the observed patterns of MC error for FE. The CC error was possibly due to inertial effects, and was more pronounced for the stair descent due to larger vertical movement and acceleration. This study led further bases for the activity-dependent correction of STA. This has the potential of improving the accuracy of motion capture and boosting its use for motion analysis as opposite to video-fluoroscopy techniques, which deliver radiation dose to the subjects

Objectives. The high revision rates of the DePuy Articular Surface Replacement (ASR) and the DePuy ASR XL (the total hip arthroplasty (THA) version) have led to questions over the viability of metal-on-metal (MoM) hip joints. Some designs of MoM hip joint do, however, have reasonable mid-term performance when implanted in appropriate patients. Investigations into the reasons for implant failure are important to offer help with the choice of implants and direction for future implant designs. One way to assess the performance of explanted hip prostheses is to measure the wear (in terms of material loss) on the joint surfaces. Methods. In this study, a coordinate measuring machine (CMM) was used to measure the wear on five failed cementless Biomet Magnum/ReCap/ Taperloc large head MoM THAs, along with one Biomet ReCap resurfacing joint. Surface roughness measurements were also taken. The reason for revision of these implants was pain and/or adverse reaction to metal debris (ARMD) and/or elevated blood metal ion levels. Results. The mean wear rate of the articulating surfaces of the heads and acetabular components of all six joints tested was found to be 6.1 mm. 3. /year (4.1 to 7.6). The mean wear rate of the femoral head tapers of the five THAs was 0.054 mm. 3. /year (0.021 to 0.128) with a mean maximum wear depth of 5.7 µm (4.3 to 8.5). Conclusion. Although the taper wear was relatively low, the wear from the articulating surfaces was sufficient to provide concern and was potentially large enough to have been the cause of failure of these joints. The authors believe that patients implanted with the ReCap system, whether the resurfacing prosthesis or the THA, should be closely monitored. Cite this article: S. C. Scholes, B. J. Hunt, V. M. Richardson, D. J. Langton, E. Smith, T. J. Joyce. Explant analysis of the Biomet Magnum/ReCap metal-on-metal hip joint. Bone Joint Res 2017;6:113–122. DOI: 10.1302/2046-3758.62.BJR-2016-0130.R2

Aims

Research on hip biomechanics has analyzed femoroacetabular contact pressures and forces in distinct hip conditions, with different procedures, and used diverse loading and testing conditions. The aim of this scoping review was to identify and summarize the available evidence in the literature for hip contact pressures and force in cadaver and in vivo studies, and how joint loading, labral status, and femoral and acetabular morphology can affect these biomechanical parameters.

Background. In vivo fluoroscopic studies have proven that femoral head sliding and separation from within the acetabular cup during gait frequently occur for subjects implanted with a total hip arthroplasty. It is hypothesized that these atypical kinematic patterns are due to component malalignments that yield uncharacteristically higher forces on the hip joint that are not present in the native hip. This in vivo joint instability can lead to edge loading, increased stresses, and premature wear on the acetabular component. Objective. The objective of this study was to use forward solution mathematical modeling to theoretically analyze the causes and effects of hip joint instability and edge loading during both swing and stance phase of gait. Methods. The model used for this study simulates the quadriceps muscles, hamstring muscles, gluteus muscles, iliopsoas group, tensor fasciae latae, and an adductor muscle group. Other soft tissues include the patellar ligament and the ischiofemoral, iliofemoral, and pubofemoral hip capsular ligaments. The model was previously validated using telemetric implants and fluoroscopic results from existing implant designs. The model was used to simulate theoretical surgeries where various surgical alignments were implemented and to determine the hip joint stability. Parameters of interest in this study are joint instability and femoral head sliding within the acetabular cup, along with contact area, contact forces, contact stresses, and ligament tension. Results. During swing phase, it was determined that femoral head pistoning is caused by hip capsule laxity resulting from improperly positioned components and reduced joint tension. At the point of maximum velocity of the foot (approximately halfway through), the momentum of the lower leg becomes too great for a lax capsule to properly constrain the hip, leading to the femoral component pistoning outwards. This pistoning motion, leading to separation, is coupled with a decrease in contact area and an impulse-like spike in contact stress (Figure 1). During stance phase, it was determined that femoral head sliding within the acetabular cup is caused by the proprioceptive notion that the human hip wants to rotate about its native, anatomical center. Thus, component shifting yields abnormal forces and torques on the joint, leading to the femoral component sliding within the cup. This phenomenon of sliding yields acetabular edge-loading on the supero-lateral aspect of the cup (Figure 2). It is also clear that joint sliding yields a decreased contact area, in this case over half of the stable contact area, corresponding to a predicted increase in contact stress, in this case over double (Figure 2). Discussion. From our current analysis, the causes and effects of hip joint instability are clearly demonstrated. The increased stress that accompanies the pistoning/impulse loading scenarios during swing phase and the supero-lateral edge-loading scenarios during stance phase provide clear explanations for premature component wear on the cup, and thus the importance of proper alignment of the THA components is essential for a maximum THA lifetime. For any figures or tables, please contact authors directly

Introduction. Native hip joint infection can result in hip arthrosis as a complication requiring the need for subsequent arthroplasty. There is little evidence to support single or staged THA. We present the results of patients who have undergone total hip arthroplasty (THA) following hip joint sepsis in our institution. Methods. Patients receiving a THA following previous hip joint sepsis between 2003 and 2015 were identified from the operative records database. Further clinical information was collected from the medical records, including timing of infection and surgery, as well as culture results. Functional outcome (Oxford Hip Score, OHS) and patient satisfaction scores (PSS) were recorded. Results. There were 21 males and 12 females with a mean age of 54 (21–82) at a mean follow up of 6.4 years (2–12) following definitive implantation. Thirteen patients underwent a single stage THA and 20 patients had a 2 stage procedure, two of whom required a repeat 1. st. stage prior to implantation. The mean time from native hip infection to operation was 17.2 years (3–59). At implantation, positive cultures were present in 19 cases, 13 of which were S. aureus. Two patients required revision due to a periprosthetic fracture and one patient required debridement, antibiotics and implant retention (DAIR). Therefore, the implant survivorship was 94% and infection rate was 3% at 6.4 years. Discussion. Both single and staged arthroplasty provide good results in patients who have developed septic arthritis. THA as a single stage was performed predominately in patients with a long remission between their initial infection and presentation. Positive culture at implantation does not confer a bad outcome as long as a thorough debridement is performed as routine in this patient group. Patient comorbidities are an important factor as both cases requiring reoperation were in IV drug users with Hepatitis C

INTRODUCTION. The purpose of this study is to elucidate longitudinal kinematic changes of the hip joint during heels-down squatting after THA. METHODS. 66 patients with 76 primary cementless THAs using a CT-based navigation system were investigated using fluoroscopy. An acetabular component and an anatomical femoral component were used through the mini-posterior approach with repair of the short rotators. The femoral head size was 28mm (9 hips), 32mm (12 hips), 36mm (42 hips), and 40mm (12 hips). Longitudinal evaluation was performed at 3 months, 1 year, and 2≤ years postoperatively. Successive hip motion during heels-down squatting was recorded as serial digital radiographic images in a DICOM format using a flat panel detector. The coordinate system of the acetabular and femoral components based on the neutral standing position was defined. The images of the hip joint were matched to 3D-CAD models of the components using a2D/3D registration technique. In this system, the root mean square errors of rotation was less than 1.3°, and that of translation was less than 2.3 mm. We estimated changes in the relative angle of the femoral component to the acetabular component, which represented the hip ROM, and investigated the incidence of bony and/or prosthetic impingement during squatting (Fig.1). We also estimated changes in the pelvic posterior tilting angle (PA) using the acetabular component position change. In addition, when both components were positioned most closely during squatting, we estimated the minimum angle (MA) up to theoretical prosthetic impingement as the safety margin (Fig.2). RESULTS. No prosthetic or bony impingement and no dislocation occurred in any hips. The mean maximum hip flexion ROM was 92.4° (range, 76.6° – 107.9°) at 3 months, 103.4° (range, 81.5° – 115.2°) at 1 year, and 102.4° (range, 87.1° – 120.6°) at 2≤ years (3 months vs 1 year, p<0.05; 1 year vs 2≤ years, p>0.05, paired t-test). The mean PA was 26.7° (range, 0.9° – 49.8°) at 3 months, 21.7° (range, 3.4° – 43.8°) at 1 year, and 21.2° (range, −0.7° – 40.4°) at 2≤ years (3 months vs 1 year, p<0.05; 1 year vs 2≤ years, p>0.05). The mean flexion ROM and MA at 2≤ years were 98.4±20.8° and 14.3±7.3° in 28 mm heads, 102.3±10.7° and 15.6±4.8° in 32 mm heads, 102.8±14.5° and 20.3±9.6° in 36 mm heads, and 103.2±16.9° and 23.4±10.9° in 40 mm heads, respectively. There were no significant differences in the hip flexion ROM between 28, 32, 36, and 40 mm head cases, whereas MA significantly increased as the femoral head diameter was larger (p<0.05, unpaired t-test). DISCUSSION AND CONCLUSION. Three-dimensional assessment of dynamic squatting motion after THA using the 2D/3D registration technique enabled us to elucidate longitudinal kinematic change of the hip joint. Longitudinal kinematic analysis indicated that hip flexion ROM and posterior tilt during squatting changed significantly by 1 year postoperatively, and there were no significant changes after 1 year while safety margin kept > 10°. For figures/tables, please contact authors directly.

Introduction. Advantages of ceramic materials for hip joint prosthesis are recognized to be high hardness, scratch resistance, improved wettability, lower friction and lower wear than CoCr surfaces. Recent studies suggest the use of ceramic femoral head reduce fretting corrosion at stem taper junction compared to metal-on-metal taper junction[1]. Continuous improvement of ceramic materials for orthopedic lead to the development of a resurfacing ceramic-on-ceramic hip joint prosthesis. The main differences of resurfacing heads respect to standard heads are their anatomical dimension and the empty shape suitable to cover the femoral bone and to connect with the resurfacing stem. Ceramic is essentially a brittle material and its strength is influenced by the minimum thickness in the stressed area. Ceramic resurfacing head minimum thickness is comparable with ceramic revision head already on the market. The aim of this study is to develop a mechanical pre-clinical analysis verification process for the newly developed system. Materials and methods. The empty shape of the ceramic resurfacing head may influence its strength in a crush loading scenario. Although this is not a physiological condition this test represents the most severe loading for a resurfacing head. Also comparative analysis can be done considering the yield point of conventional metal resurfacing heads reported by the FDA Center for Devices and Radiological Health. For this reason a static unsupported head strength test is performed by applying a compressive load perpendicular to the head axis along the equatorial plane[2](Fig.1). Resurfacing ceramic head made in ZTA is suitable both for a resurfacing stem and an adaptor to be coupled with a standard stem. Mechanical test was performed on worst case resurfacing head size both with resurfacing stem and standard stem based and on FE non linear analysis performed in ANSYS 17.2 according the following material properties: ZTA ceramic (modulus of elasticity E, Poisson ratio ν and density ρ of 348GPa, 0.23 and 4.25g/cm. 3. respectively), and Ti6Al4V (E=114GPa, ν=0.33 and ρ=4.43g/cm. 3. ). For comparison purposes unsupported test was performed on standard head Ø28#S both in Biolox®Delta and Biolox®Forte ceramic. At least three components were used for each test and the average values was compared with predicates[2]. Static compressive load was applied with MTS hydraulic actuators with load cell of 100kN. Results. FE analysis indicated the larger resurfacing head as the worst case size in the size range(Fig.2). Static unsupported head strength test was performed on resurfacing ceramic head Ø57 coupled both with resurfacing stem and standard stem, Biolox®Delta head Ø28#S, Biolox®Forte head Ø28#S and respectively reached a strength value of 53±7kN, 90±3kN, 78±27kN, 49±1kN. Equivalent test were reported in literature for DeltaSurf® Ø58 and BHR. TM. , and respectively reached a strength of 26kN and 5.6kN. Discussion and Conclusion. LIMA ceramic resurfacing head and Biolox Forte Ø28#S showed equivalent unsupported head burst strength. LIMA ceramic resurfacing head showed higher unsupported head burst strength respect to DeltaSurf® and BHR. TM. highlighting its potential in clinical use. For any figures or tables, please contact the authors directly