Aim. The aim was to investigate the value of quantitative histological analysis in the diagnosis of fracture-related infection (FRI). Patients and Methods. The clinical features, microbiology culture results and histological analysis in 156 surgically treated non-unions were used to stratify the likelihood of associated infection. There were 64 confirmed infected non-unions (≥1 confirmatory criteria; pus, sinus and bacterial growth in ≥2 samples), 66 aseptic non-unions (no confirmatory criteria) and 26 possibly infected (pathogen identified from a single specimen and no confirmatory criteria). The histological inflammatory response was assessed by average neutrophil polymorphs (NPs) counts per high power field (HPF) and compared to the established diagnosis. Results. Assuming a cut-off of >5NPs/HPF for positive histological diagnosis, there was 80% sensitivity and 100% specificity (accuracy 90%). Using a cut-off of any NPs/HPF (>0) for negative histological diagnosis there was a sensitivity of 98% and a specificity of 85% (accuracy 92%). Conclusion. Histology can be used in a bimodal fashion as a diagnostic test for FRI. The presence of >5 NPs/HPF has a positive predictive value of 100%, while the complete absence of any NPs is almost always indicative of an aseptic non-union (negative predictive value: 98%)

Aims. This study aimed to investigate the role of quantitative histological analysis in the diagnosis of fracture-related infection (FRI). Patients and Methods. The clinical features, microbiology culture results, and histological analysis in 156 surgically treated nonunions were used to stratify the likelihood of associated infection. There were 64 confirmed infected nonunions (one or more confirmatory criteria: pus, sinus, and bacterial growth in two or more samples), 66 aseptic nonunions (no confirmatory criteria), and 26 possibly infected nonunions (pathogen identified from a single specimen and no confirmatory criteria). The histological inflammatory response was assessed by average neutrophil polymorph (NPs) counts per high-power field (HPF) and compared with the established diagnosis. Results. Assuming a cut-off of over five neutrophils per high-power field to diagnose septic nonunion, there was 80% sensitivity and 100% specificity (accuracy 90%). Using a cut-off of no neutrophils seen in any high-power field to diagnose aseptic nonunion, there was a sensitivity of 85% and a specificity of 98% (accuracy 92%). Conclusion. Histology can be used in a bimodal fashion as a diagnostic test for FRI. The presence of more than five NPs/HPF had a positive predictive value for infected nonunion of 100%, while the complete absence of any NPs is almost always indicative of an aseptic nonunion (positive predictive value of 98%). Cite this article: Bone Joint J 2018;100-B:966–72

Aims

This study aimed to investigate the clinical characteristics and outcomes associated with culture-negative limb osteomyelitis patients.

The Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score was developed in 2004 to risk-stratify patients with soft tissue infections using common blood tests when the clinical picture is equivocal. A score ≥ 6 conferred a positive predictive value (PPV) of 92% and negative predictive value of 96% for necrotising fasciitis. We retrospectively calculated LRINEC scores for Orthopaedic patients admitted to ITU in our hospital with limb soft-tissue infection and confirmed Group-A Streptococcus or Staphylococcus in fluid, blood, tissue or swab culture between 2010–2017 (n=10). Mean age = 57.4 and 60% were female. Half of all patients died during admission. Mean LRINEC score of all patients was 5.3±3.1 (median = 6). Mean score in deceased patients was 4.8±2.8 (scores: 0,5,6,6,7; median = 6); in discharged patients mean = 5.8±3.7 (scores: 0,5,7,7,10; median = 7). 6 patients had a score ≥6, making our PPV 60%. 4 patients had necrotising fasciitis confirmed on histology (LRINEC scores = 0,5,7,10). Our PPV of 60% is less than the figure obtained in the original paper. 2 patients with a LRINEC score <6 died during admission, including a patient with a score of 0. Furthermore, a patient with necrotising fasciitis confirmed on histology also had a LRINEC score of 0. We conclude that LRINEC scores should not delay surgery when clinical suspicion is high, and should be used as an adjunct to clinical decision-making, rather than a replacement, as patients with low LRINEC scores can also have confirmed necrotising fasciitis and poor outcomes

Osteoinductive bone substitutes are in their developmental infancy and a paucity of effective grafts options persists despite clinical demand. Bone mineral substitutes such as hydroxyapatite cause minimal biological activity when compared to osteoinductive systems present biological growth factors in order to drive bone regeneration. We have previously demonstrated the in-vitro efficacy of a bioengineered system at presenting growth factors at ultra low-doses. This study aimed to translate this growth factor delivery system towards a clinically applicable implant. Osteoinductive surfaces were engineered using plasma polymerisation of poly(ethyl acrylate) onto base materials followed by adsorption of fibronectin protein and subsequently growth factor (BMP-2). Biological activity following ethylene oxide (EO) sterilisation was evaluated using ELISAs targeted against BMP-2, cell differentiation studies and atomic force microscopy. Scaffolds were 3D printed using polycaprolactone/hydroxyapatite composites and mechanically tested using a linear compression models to calculate stress/strain. In-vivo analysis was performed using a critical defect model in 23 mice over an 8 week period. Bone formation was assessed using microCT and histological analysis. Finally, a computer modelling process was developed to convert patient CT images into surface models, then formatted into 3D-printable scaffolds to fill critical defects. Following EO sterilisation, there was no change in scaffold surface and persistent availability of growth factors. Scaffolds showed adequate porosity for cell migration with mechanical stiffness similar to cancellous bone. Finally, the in vivo murine model demonstrated rapid bone formation with evidence of trabecular remodelling in samples presenting growth factors compared to controls

Fracture repair occurs by two broad mechanisms: direct healing, and indirect healing with callus formation. The effects of bisphosphonates on fracture repair have been assessed only in models of indirect fracture healing. A rodent model of rigid compression plate fixation of a standardised tibial osteotomy was used. Ten skeletally mature Sprague–Dawley rats received daily subcutaneous injections of 1 µg/kg ibandronate (IBAN) and ten control rats received saline (control). Three weeks later a tibial osteotomy was rigidly fixed with compression plating. Six weeks later the animals were killed. Fracture repair was assessed with mechanical testing, radiographs and histology. The mean stress at failure in a four-point bending test was significantly lower in the IBAN group compared with controls (8.69 Nmm. -2. (. sd. 7.63) vs 24.65 Nmm. -2. (. sd. 6.15); p = 0.017). On contact radiographs of the extricated tibiae the mean bone density assessment at the osteotomy site was lower in the IBAN group than in controls (3.7 mmAl (. sd. 0.75) vs 4.6 mmAl (. sd. 0.57); p = 0.01). In addition, histological analysis revealed progression to fracture union in the controls but impaired fracture healing in the IBAN group, with predominantly cartilage-like and undifferentiated mesenchymal tissue (p = 0.007). . Bisphosphonate treatment in a therapeutic dose, as used for risk reduction in fragility fractures, had an inhibitory effect on direct fracture healing. We propose that bisphosphonate therapy not be commenced until after the fracture has united if the fracture has been rigidly fixed and is undergoing direct osteonal healing. Cite this article: Bone Joint J 2013;95-B:1263–8

Aims. The Intraosseous Transcutaneous Amputation Prosthesis (ITAP) may improve quality of life for amputees by avoiding soft-tissue complications associated with socket prostheses and by improving sensory feedback and function. It relies on the formation of a seal between the soft tissues and the implant and currently has a flange with drilled holes to promote dermal attachment. Despite this, infection remains a significant risk. This study explored alternative strategies to enhance soft-tissue integration. Materials and Methods. The effect of ITAP pins with a fully porous titanium alloy flange with interconnected pores on soft-tissue integration was investigated. The flanges were coated with fibronectin-functionalised hydroxyapatite and silver coatings, which have been shown to have an antibacterial effect, while also promoting viable fibroblast growth in vitro. The ITAP pins were implanted along the length of ovine tibias, and histological assessment was undertaken four weeks post-operatively. Results. The porous titanium alloy flange reduced epithelial downgrowth and increased soft-tissue integration compared with the current drilled flange. The addition of coatings did not enhance these effects. Conclusion. These results indicate that a fully porous titanium alloy flange has the potential to increase the soft-tissue seal around ITAP and reduce susceptibility to infection compared with the current design. Cite this article: Bone Joint J 2017;99-B:393–400

Introduction. Traumatized musculoskeletal tissue often exhibits prolonged time to healing, mostly due to low blood flow and innervation. Intermittent Pneumatic Compression (IPC) increases blood flow and decreases thromboembolic event after orthopedic surgery,[1] however little is known about healing effects.[2] We hypothesized that IPC could stimulate tissue repair: 1.) blood flow 2.) nerve ingrowth 3.) tissue proliferation and during immobilisation enhance 4.) biomechanical tissue properties. Methods. Study 1: In 104 male Sprague Dawley (SD) rats the right Achilles tendon was ruptured and the animals freely mobilized. Half the group received daily IPC-treatment, using a pump and cuff over the hindpaw that inflates/deflates cyclicly, 0–55mmHg (Biopress SystemTM, Flexcell Int.), and the other half received sham-treatment. Healing was assessed at 1,3,6 weeks by perfusion-analysis with laser doppler scanner (Perimed, Sweden), histology and biomechanical testing. Study 2: 48 male SD-rats were ruptured as above. Three groups of each 16 rats were either mobilized, immobilized or immobilized with IPC treatment. Immobilization was performed by plaster cast. Healing was assessed at 2 weeks with histology and biomechanical testing. Results. Study 1: At 3 and 6 weeks reperfusion increased by 21% and 23% (p< 0.05) after IPC-treatment, strengthened by the observation of elevated numbers of blood vessels and nerves. Fibroblast density was at all time points significantly increased in the IPC group. At three and six weeks the IPC treated tendons displayed an increased tissue organization confirmed by higher collagen I/III ratio in the IPC group. No differences (p = 0.10) were found regarding biomechanical strength. Study 2: Compared to mobilization, immobilization caused a downregulation (p<0.05) of all biomechanical and histological parameters, eg. maximum force decreased 80% and collagen III occurrence by 83%. However when immobilization was combined with IPC biomechanical and histological healing increased significantly compared to pure immobilization, eg. maximum force increased 63% and collagen III occurrence by 150%. Conclusion. This study demonstrated that IPC treatment can counteract biomechanical and morphological deficits caused by immobilization by enhancing proliferative soft tissue repair. Thus, IPC promotes tissue repair by stimulating tissue perfusion and nerve ingrowth as well as accelerating both fibroblast proliferation and collagen organization

Aims

Monocyte-lymphocyte ratio (MLR) or neutrophil-lymphocyte ratio (NLR) are useful for diagnosing periprosthetic joint infection (PJI), but their diagnostic values are unclear for screening fixation-related infection (FRI) in patients for whom conversion total hip arthroplasty (THA) is planned after failed internal fixation for femoral neck fracture.

Introduction. Modular tantalum augments have been introduced to manage severe bone defects in hip and knee revision surgery. The porous surfaces of tantalum augments are intended to enhance osseointegration and a number of studies have documented their excellent biocompatibility. However, the characteristics of tantalum augment osseointegration on human ex vivo specimens from re-revision procedures have not been reported so far. Methods. Out of a total number of 324 hip and knee revisions with a tantalum augment performed in our institution between 2007 and 2010 four patients had to be re-revised at a mean followup time of 15 months. The causes for re-revision were a periprosthetic acetabular fracture in one, a loosening of a tibial component in one and periprosthetic hip infections in two cases. To characterize osseointegration of the tantalum augments, they were removed during revision surgery and subjected to undecalcified processing. All specimens were analysed by contact radiography, histology (toluidine blue, von Kossa) and quantitative histomorphometry. Results. In all specimens trabecular ingrowth was apparent along the former bone-augment-interface. The depth of bone ingrowth into the porous microarchitecture of the augments reached up to 2000 μm. Thin-section analysis revealed scattered and partially mineralized bone forming units within the interior of the augments. Conclusions. To the best of our knowledge this is the first histomorphometric analysis on the osseointegration of tantalum augments in human ex vivo specimens. In the presented series porous tantalum showed excellent osteoconductive characteristics on the histological level. These early ex vivo histological findings are promising, but structural analysis of later re-revision cases is needed

Background. An increased incidence of periprosthetic osteolysis, resulting in loss of biologic fixation, has been recently reported in contemporary THAs with low-carbide metal-on-metal compared to metal-on-polyethylene couple bearings. A hypersensitivity reaction due to Co and Cr debris is reported as a potential cause for failure of THAs with high-carbide bearings, but there are no evidence-based data for this reaction in low-carbide metal-on-metal bearings. Questions/purposes. We investigated whether there were differences in immunologic hypersensitivity reactions in retrievals from revised THAs with COP versus MOM bearing couples. Patients and Methods. We compared newly formed capsule and periprosthetic interface membranes retrieved from revision surgery due to aseptic failure in 20 patients with low-carbide bearings and 13 patients with ceramic-on-polyethylene bearings. For control tissue we obtained samples from the hip capsule during the primary THA implantation in 13 patients with low-carbide bearings and seven with ceramic-on-polyethylene. We examined the tissues with conventional histologic and immunohistochemical methods. Results. Compared to the controls and the tissue from patients with ceramic-on-polyethylene bearings, the tissues from patients with low-carbide metal-on-metal bearings were associated with (1) extensive necrosis and fibrin exudation in the newly formed hip capsule and (2) diffuse and perivascular lymphocytic infiltration of a higher degree than in the ceramic-on-polyethylene hips in conventional histologic examination and (3) more T than B cells. Conclusions. The conventional histologic and immunohistochemical findings in tissues retrieved from failed THAs with low-carbide metal-on-metal bearings are consistent with a link between hypersensitivity and osteolysis with low-carbide bearing couple THA

Posterolateral spinal fusion (PSLSF) in rabbits is a challenging model for bone substitutes because the transverse processes are extremely thin and the space to be filled with bone is greater than critical and meiopragic in terms of vascularity. Several investigators have shown beneficial effects of PRP in bone and soft-tissue healing processes. However, controversial results have been reported in clinical setting analysing the effectiveness of PRP. Aim of the present study was to test the effectiveness of PRP in experimental model of PLSF in rabbits. MATERIAL AND METHODS. 20 White females New Zeland Rabbits were used. Seven rabbits (Group 1) had PRP plus carrier on the right side (Group 1A) and plus carrier and fresh bone marrow on the left side (Group 1B). Seven rabbits (Group 2) had carrier alone on the right side (Group 2A) and carrier plus fresh bone marrow on the left side (Group 2B). Six rabbits (Group 3) had sham operation on both right and left sides. Animals were sacrificed 6 months after surgery and the lumbar spine submitted to radiolographic and histologic analysis. Vascular density (VD) was also assessed in the different zone of the grafted material. RESULTS. Radiographs showed a complete fusion in 83% of group 1A and in 83% of group 1B, and in 86% of group 2A and 2B. Pseudarthrosis or non union, was observed in 1 specimen of group 1B and 2A and in all specimens of group 3 (sham). In contrast to radiographic results, no specimen showed a complete bony bridge between the transverse processes on histologic analysis. VD was significantly greater in the periapophyseal compared to the interapophyseal region of the graft material. However, no significant difference was found in the VD between groups. CONCLUSIONS. In this study PRP alone, or augmented with fresh bone marrow, failed to induce a histologically proved bony fusion in the PLSF model. Factors which may influence the effectiveness of PRP should be further addressed before applying PRP in the clinical setting

Aims

Pin-site infection remains a significant problem for patients treated by external fixation. A randomized trial was undertaken to compare the weekly use of alcoholic chlorhexidine (CHX) for pin-site care with an emollient skin preparation in patients with a tibial fracture treated with a circular frame.

Introduction. The hematoma occurring at a fracture site is known to play an important role in fracture healing. Previously, we demonstrated that fracture hematoma contained multilineage mesenchymal progenitor cells. On the other hand, the process of fracture healing is associated by two different mechanisms, intramembranous and endochondral. However, there are no reports proving the details about cellular analysis in the process of endochondoral ossification. Hypothesis. We hypothesized that one of the cell origins for endochondral ossification after fracture was hematoma. Materials & Methods. Fracture hematoma was obtained during osteosynthesis. Hematoma-derived cells were isolated and cultured for 5-weeks of chondrogenic induction followed by 2-weeks hypertrophic induction using pellet culture system. The pellets were analyzed histologically and immunohistochemically. The gene expression levels of chondrogenic, hypertrophic, osteogenic and angiogenic markers were measured by real-time PCR. Results. The histological and immunohistochemical analysis revealed that the Hematoma-derived cells differentiated into hypertrophic chondrocytes through chondrocytes, and finally differentiate into calcifying chondrocytes. The same trend was seen in the gene expression using real-time PCR analysis. Discussion & Conclusions. Our results suggest that fracture hematoma may be an origin of cells which play key roles in the process of endochondoral ossification during fracture healing

Introduction. what size of defect is optimal for creating an atrophic nonunion animal model has not been well defined. Our aim in this study was to establish a clinically relevant model of atrophic nonunion in rat femur by creation of a bone defect to research fracture healing and nonunion. Materials and methods. We used 30 male Fischer 344 rats (aged 10–11 weeks), which were equally divided into six groups. The segmental bone defects to a single femur in each rat were performed by double transverse osteotomy, and different sized defects were created by group for each group (1 mm, 2 mm, 3 mm, 4 mm, 5 mm and 6 mm). The defects were measured and maintained strictly by using an original external fixator. The periosteum for each defect was stripped both proximally and distally. Thereafter, these models were evaluated by radiology and histology. Radiographs were taken at baseline and at intervals of two weeks over a period of 8 weeks. Atrophic nonunion was defined as a lack of continuity and atrophy of both defect ends radiologically and histologically at eight weeks. Results. In the 1 mm defect group, all defects had healed. In the 2 mm group, one-fifth remained atrophic nonunion. In the 3 mm group, three-fifths had atrophic nonunion, and all of the defects of groups of 4 mm and over were atrophic nonunion. Conclusion. This study showed that we were able to predictively produce an atrophic nonunion animal model by creating defects of at least 4 mm in the Fischer 344 rat femur

Reconstruction of 10mm segmental bone defects in rat by mesenchymal stem cell derived chondrogenic cells (MSC-DC). Background. Mesenchymal stem cell derived condrogenic cells (MSC-DC) have excellent potential for healing 5 mm bone defect in rat femur. Purpose. To evaluate the effectiveness of MSC-DC on bone healing in 10 mm segmental bone defects in rat femur. Methods. 10 millimeter bone defects were produced in rat femur and fixed with external fixator. We divided this model into four groups according to the kind of graft for bone defects. These bone defects were grafted by MSC-DC seeded on a poly (DL-lactic acid-co-glycolic acid) (PLGA) scaffold in Group A, MSC seeded on a PLGA scaffold in Group B, PLGA scaffold only in Group C, and autologus bone graft in Group D. The healing processes were monitored radiographically and studied biomechanically and histologically. Results. All the bone defects in Group A healed radiographically with bridging callus formation at 4 weeks after the procedure, while none of Group B, C, and D had achieved bone union even at 8 weeks. Mechanical testing revealed that Group A showed approximately 40 % bending strength at 4 weeks compared with the contralateral side, and approximately 60 % at 8 weeks. In histology, Group A, maturation of bridging callus occurred from outside and enchondral ossification was prominent from inside. Conclusion. This study showed that MSC-DC with PLGA scaffold enhances bone healing even in large bone defects

Aims

The aim of this study was to evaluate the diagnostic value of preoperative serum CRP, white blood cell count (WBC), percentage of neutrophils (%N), and neutrophil to lymphocyte ratio (NLR) when using the fracture-related infection (FRI) consensus definition.

Background. Pseudotumours have been associated with metal-on-metal (MoM) hip replacements. We define it as a solid mass which may have cystic components that is neither neoplastic nor infectious in aetiology. The cause of a pseudotumour is not fully understood but could be due to excessive wear, metal hypersensitivity or due to an as-yet unknown cause. Aim. We present the retrieval analysis of early failure MoM hips revised for pain, loosening or a symptomatic mass. Tissues and implants were examined for the possible causes of failure and pseudotumour formation. Corrosion as a potential new cause for pseudotumour formation will be presented. Methods. A group of 16 MoM hip replacements were collected for retrieval analysis. Six of which had a pseudotumour. An Artificial Hip Profiler (Redlux Ltd) was used to measure wear. Edge loading was determined using the 3D wear data. Tissues were histologically evaluated using a 10-point ALVAL scoring system, which strongly suggests hypersensitivity (1). Cases were assigned to one of three categories: high wear (rates >5m/yr), hypersensitivity or corrosion. Results. Of the 6 pseudotumours, 3 had edge loading, 2 had high ALVAL scores and one had corrosion at the head taper junction. The high wear group, (3 cases) all demonstrated edge loading. Histology revealed more metal wear particles and macrophages, with a low ALVAL score in these cases. Two cases were found to have hypersensitivity with a high ALVAL score, more lymphocytes with less visible wear debris and macrophages. The wear rates were also lower, <4m/yr. One total hip replacement had corrosion at the junction between the head and adaptor taper for the stem. The bearings had low wear and the tissue had extensive necrosis as well as products of corrosion enclosed in fibrin (2). There was no suggestion of hypersensitivity (low ALVAL score). Conclusion. Pseudotumours can be caused by high wear, but if this is ruled out, a hypersensitivity or corrosion product reaction should be considered

INTRODUCTION. Surgical correction of spinal deformities in the growing child can be applied with or without fusion. Sublaminar wiring, first described by Luque, allows continuation of growth of the non-fused spine after correction of the deformity. Neurological complications and wire breakage are the main clinical problems during the introduction and removal of currently used sublaminar wires. In this pilot study a posterior hybrid construction with the use of a medical-grade UHMWPE (Dyneema Purity®) sublaminar wire was assessed in an ovine model. We hypothesized that such a hybrid construction can safely replace current titanium laminar wires, while providing sufficient stability of the non-fused spinal column with preservation of growth. MATERIALS AND METHODS. This study included 6 Tesselaar sheep, age 7±2months. Two pedicle screws (Legacy system, Medtronic) were placed at lumbar level. Four consecutive laminae were attached to two titanium bars (4.5 mm) using 3 mm diameter UHMWPE (Dyneema Purity®) on the left side and 5 mm diameter on the right side. The sublaminar wires were fixed with a double loop sliding knot and tightened with a tensioning device. As a control, in one animal titanium sublaminar wires (Atlas cable, Medtronic) were applied. After sacrifice the spine of the animals was harvested. Radiographs were taken and CT scans were performed. The vertebrae were dissected and placed in formaldehyde for macroscopic and histological evaluation. RESULTS. The animals were sacrificed after a (minimal) postoperative period of 15 weeks. One animal developed a wire fistula and one animal died the first postoperative day due to complications of the anesthesia. None of the 3 or 5 mm knots loosened and no neurological complications occurred. An average of 8.7 mm growth was seen over the segment operated on. Computed tomography confirmed the preserved stability. Even though no decortication was performed, variable bone bridges with fused levels were seen on CT. Macroscopic and histological analysis showed no inflammation at lamina and dura levels containing Dyneema Purity®, with the exception of the case with the fistula where it was observed locally. DISCUSSION. This pilot animal model study shows that the UHMWPE laminar wire made by Dyneema Purity® has good handling and tensioning properties and can provide sufficient stability in fusionless spinal instrumentation while allowing substantial growth. The examined model showed to be a feasible spinal study model, without occurrence of neurological problems. Reactive periostal bone formation with fusion levels led to some restrictions in this model. In the future it will be necessary to test the described construction in a large animal scoliosis model

Introduction. Ectopic ossification (EO) at the acetabular rim has been suggested to be associated with pincer impingement and to lead to ossification of the labrum. However, this has never been substantiated with histological, radiographic and MRI findings in large cohorts of patients. We hypothesized that it is more a bone apposition of the acetabular rim and that it occurs more frequently in coxa profunda (CP) hips. Materials and Methods. In the first part, a cohort of 20 hips with this suspected ectopic rim ossification (EO) pattern were identified. The radiographic features that could be associated with this ossification pattern were described and evaluated by a histologic examination of intra-operative samples taken from the rim trimming. In the second part, we assessed the prevalence of this ectopic ossification process in a cohort of 203 patients treated for FAI. Results. Histologic examination revealed that new acetabular bone formation was either overgrowing the non-ossified labrum or moving it away from the native rim. Radiologically, this was associated with an “indentation sign” and/or a “double line sign”. There were no specimens that had shown any evidence of labral ossification. EO was found in 26 hips (18%) of the second cohort. Twenty of 26 hips (77%) with EO had CP morphology and 29% of CP hips had EO signs. In contrast, only 6 non-profunda hips (8%) were associated with EO. There was a high correlation between XR and MRI findings as >80% of XR findings were confirmed on MRI. Sixty-nine hips had CP morphology. The double line sign (N = 13), the indentation sign (N = 12) and a prominent lateral rim (N = 11) were found. Hips with an EO pattern were found in patients that were significantly older than those without EO (p = 0.01). The acetabular characteristics of the EO groups were not significantly different from the CP hips without EO. The femoral characteristics were significantly different between groups with lower neck shaft angles (128° vs 134°;p = 0,0002) and shorter femoral necks lengths (62mm vs 65mm; p = 0,04)) in the EO group. The mean Tonnis classification was not significantly different (p = 0,18). In addition, the mean acetabular cartilage degeneration status was not different between both groups (p = 0,9). Rim trimming down to the native acetabular bone was done in all cases either by arthroscopy (N = 40) or open surgical dislocation (N = 17). Discussion. Ectopic ossification of the acetabular rim predominantly occurs in CP and is associated with specific anatomic features of the proximal femur. This type of impingement seems to be different and less aggressive than other described impingement processes. The double line sign and indentation sign are highly indicative for this EO process and are indicative for a longstanding impingement problem. Trimming of the acetabular rim should be conducted