In this paper we propose a new classification of neurogenic peri-articular heterotopic ossification (HO) of the hip based on three-dimensional (3D) CT, with the aim of improving pre-operative planning for its excision.

A total of 55 patients (73 hips) with clinically significant HO after either traumatic brain or spinal cord injury were assessed by 3D-CT scanning, and the results compared with the intra-operative findings.

At operation, the gross pathological anatomy of the HO as identified by 3D-CT imaging was confirmed as affecting the peri-articular hip muscles to a greater or lesser extent. We identified seven patterns of involvement: four basic (anterior, medial, posterior and lateral) and three mixed (anteromedial, posterolateral and circumferential). Excellent intra- and inter-observer agreement, with kappa values > 0.8, confirmed the reproducibility of the classification system.

We describe the different surgical approaches used to excise the HO which were guided by the 3D-CT findings. Resection was always successful.

3D-CT imaging, complemented in some cases by angiography, allows the surgeon to define the 3D anatomy of the HO accurately and to plan its surgical excision with precision.

Cite this article: Bone Joint J 2015; 97-B:899–904.

Pelvic discontinuity represents a rare but challenging problem for orthopaedic surgeons. It is most commonly encountered during revision total hip replacement, but can also result from an iatrogentic acetabular fracture during hip replacement. The general principles in management of pelvic discontinuity include restoration of the continuity between the ilium and the ischium, typically with some form of plating. Bone grafting is frequently required to restore pelvic bone stock. The acetabular component is then impacted, typically using an uncemented, trabecular metal component. Fixation with multiple supplemental screws is performed. For larger defects, a so-called ‘cup–cage’ reconstruction, or a custom triflange implant may be required. Pre-operative CT scanning can greatly assist in planning and evaluating the remaining bone stock available for bony ingrowth. Generally, good results have been reported for constructs that restore stability to the pelvis and allow some form of biologic ingrowth.

Cite this article: Bone Joint J 2013;95-B, Supple A:109–13.

Delays in the surgical treatment of acetabular fractures often results in extensile or combined approaches being required. This study reports the outcome from a regional centre aiming to treat these fractures via a single surgical approach where possible. Seventy-two patients (73 displaced acetabular fractures) with an average age of 39.5 years (range 15–76 years) were studied with an average follow up period of 45.5 months (range 24–96). All radiographs were reviewed together with a full clinical assessment of each patient including the Harris Hip Score. Thirty-four fractures were simple and 39 were complex including 27 both column fractures. Eight were noted to have an associated injury to the femoral head. The average time from injury to surgery was 11.7 days (range 1–35 days) with 80 percent of cases being operated on within two weeks after injury. In 67 fractures (92%), including 24 both column fractures, a single approach alone was used (Anterior Ilioin-guinal 26 cases; Posterior Kocher-Langenbeck 41 cases). Five fractures needed an extensile triradiate approach and only one case required a combined anterior and posterior approach. A congruent reduction (gap or step of 2mm or less) was achieved in 65 cases (89%). Functional outcome was good with an average Harris Hip Score of 85 (range 20–100). There were 2 cases of deep infection (2.7%) and 4 patients (5.5%) required later hip replacement. There were no cases of venous thrombosis. Twenty cases exhibited heterotopic ossification of varying degree but none of these were grade IV. Conclusion: In most cases, internal fixation of a displaced acetabular fractures is possible via a single surgical approach. Morbidity and complications are much reduced but single approach surgery requires that patients are assessed and treated early and prompt referral to a specialist unit is recommended

There are a variety of surgical approaches available for open reduction and internal fixation of acetabular fractures. Some centres have avoided the use of the triradiate approach in the belief that it may result in a significantly higher rate of heterotopic ossification. This has not been our experience. In contrast to many centres, acetabular fractures are treated in an emergent manner, with surgery usually undertaken within the first few days post injury. It is the investigators’ belief that this may in part result in a lower rate of heterotopic ossification. The triradiate approach has fallen out of favour in the treatment of acetabular fractures due to concerns with both wound healing and heterotopic ossification. This approach however has been utilised frequently at the Vancouver General Hospital (VGH) in the treatment of acetabular fractures. The purpose of this study was to review the results and complications of this approach experienced in the large series at VGH. We concluded that the results of this approach are acceptable with the exposure allowing anatomical fracture reduction in the vast majority of cases. The complication rate was low, as was the rate of heterotopic ossification. The significance of this study is to highlight that this approach remains extremely useful in the treatment of acetabular fractures, due to its ability to give excellent exposure while still having an acceptably low complication rate. We believe that the ability of our unit to operate on these injuries in an emergent manner may impart the low rate of heterotopic ossification that we have observed. There were a total of one hundred and sixty-one acetabular fractures that were treated operatively with the triradiate approach over the period 1989 to 2001. Of these, the majority were two column injuries (79 or 49%), T type fractures (34 or 21%) and transverse fractures (17 or 11%). The average age of the patients was thirty-seven years and the average time to surgery was three days. Our early complications included five cases of failure of fixation or loss of reduction of the fracture, two cases of neurovascular injury, two cases of superficial wound infection, one case of deep wound infection and one case of wound breakdown. The study involved examining patient hospital records and radiographs and included fracture types, patient ages, delay to surgery, post-operative complications and degree of fracture reduction and healing. Grading of heterotopic ossification was performed by reviewing the anteroposterior radiographs and using Gruen’s classification system

We investigated the results of complex acetabular fractures that were treated through the extended triradiate approach between January 1996 and September 2002 in our clinic. Sixty acetabular fractures were treated surgically during this period in total. Twenty-nine complex fractures that were treated through the triradiate approach with a minimum 2 years follow-up included in the study. The mean patient age was 43 years. There were 10 both column, 9 T shaped, 2 anterior column – posterior hemitransverse, 4 transverse with comminuted roof area, 5 posterior wall with comminuted roof area and 1 posterior column posterior wall fractures. Associated injuries were 2 full-thickness chondral injury of the head, one Pipkin type II fracture, 5 posterior and one central dislocation of the ipsilateral femoral head; and in 4 hips acetabular marginal impaction. The average follow-up was 63.2 months. The postoperative reduction was graded as excellent in 72.4 % and imperfect in 6.9 % of the patients. The hips were evaluated functionally according to the modified Postel D’Aubigne score and rated as excellent in 10 patients (34.5 %), good in 14 patients (48.2 %), fair in 3 patients (10.3 %) and poor in 2 patients (6.9 %). There were 2 deep infections (6.9 %), 2 avascular necrosis of the head (6.9 %), and 4 (13.8 %) non-disabling heterotopic ossification. We beleive that triradiate approach provides good visualization for anatomical reduction of the complex acetabular fractures, but the surgeon should be aware of its possible complications

There remains uncertainty about the most effective surgical approach in the treatment of complex fractures of the acetabulum. We have reviewed the experience of a single surgeon using the extended iliofemoral approach, as described by Letournel.

A review of the database of such fractures identified 106 patients operated on using this approach with a minimum follow-up of two years. All data were collected prospectively. The fractures involved both columns in 64 (60%). Operation was undertaken in less than 21 days after injury in 71 patients (67%) and in 35 (33%) the procedure was carried out later than this. The reduction of the fracture was measured on plain radiographs taken after operation and defined as anatomical (0 to 1 mm of displacement); imperfect (2 to 3 mm) or poor (> 3 mm). The functional outcome was measured by the modified Merle d’Aubigné and Postel score. The mean follow-up was for 6.3 years (2 to 17).

All patients achieved union of the fractures. The reduction was graded as anatomical in 76 (72%) of the patients, imperfect in 23 (22%), and poor in six (6%). The mean Merle d’Aubigné and Postel score was 15 (5 to 18) with 68 patients (64%) showing good or excellent and 38 (36%) fair or poor results. Function correlated significantly with the accuracy of the reduction (p < 0.009). Significant heterotopic ossification developed in 32 patients (30%) and was associated with a worse mean Merle d’Aubigné and Postel score of 13.7.

The extended iliofemoral approach can be performed safely in selected complex acetabular fractures with an acceptable clinical outcome and rate of complications. Effective prophylaxis against heterotopic ossification should be strongly considered.

Introduction and Aims: The use of the triradiate approach has been associated with high rates of wound dehiscence, wound infection and significant heterotopic ossification. This approach has been the favoured extensile exposure at the Vancouver General Hospital for many years. We will review the outcomes of the use of this approach in the treatment of acetabular fractures. Method: Patients were identified from the database at Vancouver General Hospital who had their acetabular fractures treated through a triradiate approach from the period January 1989 through to December 2001. Patients with a delay of greater than three weeks from injury were excluded. A retrospective review of the hospital and out-patient records and all available radiographs was performed. Patients were contacted to determine if they required any further surgery and to assess their current functional status with appropriate outcome scores. Patients were also invited to undergo repeat radiographic assessment. Results: Of a total of 407 acetabular fractures treated surgically, 152 open reductions were performed through the triradiate approach. The average age of these patients was 38 years and 114 (75%) of these were male. Patients referred from other hospitals totalled 128 (84%). Wound outcomes were known in 138 cases. Wound complications included five cases of wound dehiscence, of which four resolved with no undue effects. There were three cases of superficial wound infection and five cases of deep wound infection. Two of the patients with deep wound infection had sustained compound acetabular injuries and a further two had significant risk factors for infection (septicaemia from chest infection and significant soft tissue necrosis). Trochanteric osteotomy was performed in 139 (91%) cases. There were only two cases of trochanteric non-union in this series, however 21 cases required removal of painful trochanteric screws. With respect to heterotopic ossification, there was a 15% Broker III/IV incidence. In this group, the injury severity scores were higher, there was a greater delay to surgery and there was a greater need for mechanical ventilation due to multiple injuries. In the group, 24 hip reconstructions were required over the period. Conclusion: In our centre, we found a low rate of wound dehiscence and deep wound infection associated with the triradiate approach in the treatment of acetabular fractures. Both open acetabular fractures developed deep infection. Trochanteric irritation was a problem in a number of the patients. The rate of significant hetero-topic ossification was low

Aim: We report results of surgical treatment of acetabular fractures and highlight the importance of single approach for complex fractures. Methods: 50 acetabular fractures referred to a specialist unit between 1994 and 1999 were treated surgically with anatomic reduction and internal þxation. Mean follow up was 32.3 months (14 to 67). Patients were regularly followed up in a special pelvic clinic for documentation of Harris hip score (pain, function, movement, activity), and radiological evidence of healing, avascular necrosis or other complications. Results: 18 patients were treated with the anterior ilioinguinal approach for 11 both- column, 3 anterior column, 3 transverse fractures and 1 central dislocation. 28 patients had posterior Kocher-Langenbeck approach for 17 posterior fracture dislocations, 2 both-column, 6 posterior wall and 1 each of transverse, posterior column and Tshaped fracture. 4 needed combined anterior- posterior or extensile triradiate approach due to comminution or delayed referral. Mean hospital stay was 24.7 days and mean injury surgery interval was 10.8 days. Of the 42 transfers from other units, 7 had surgery after 3 weeks from the injury. Outcome: 47 patients were followed up. Mean Harris hip score was 82.7 (31–100) for the posterior approach group, and 78.1 (27–99) for anterior approach group.3 patients needed total hip arthroplasty for secondary arthritis. Grade3–4 Brooker ectopic bone was noted in 3 posterior and 1 anterior approaches. There was no infection or avascular necrosis. Conclusion: Single approach surgery was possible in 46 patients and had a low rate of complications. Poor outcome occurred in highly comminuted fractures or with a delay in referral. Anterior ilioinguinal approach, although demanding, was the approach of choice for both column fractures. Early referral to a specialist unit is recommended

Aim: The purpose of this study is to determine the outcome in patients with acetabular fractures treated either conservatively or surgically. Method: From 1990–2000, we treated 152 patients with 158 acetabular fractures. 63 patients were treated nonoperatively and 95 operatively. According to Tile classification there were 70 type A, 52 type B, 36 type C fractures. Mean follow up was 90 months (23–151 months). Indications for surgery were fracture displacement of more than 2mm, hip joint instability, intrarticular fragments and ipsilateral femoral fracture. Surgical approaches used included the kocher-Langenbeck and the triradiate approach. Follow up consisted of radiological examination and functional assessment using Merle d’ Aubigne score. Results: 53 conservatively treated patients followed up. 39 (73.6%) had excellent and good results and 14 (26.4%) fair and poor results due to excessive fracture comminution, severe osteoporosis, or they were too sick to be operated on. 83 operatively treated patients with 85 fractures were followed-up. Anatomic reduction was achieved in 57 fractures, satisfactory in 18 and poor in 10 fractures. Functional outcome was excellent or good in 60 (72.3%) patients and fair and poor in 23 (27,7%).The complications were 3 wound infections, 4 cases of femoral head osteonecrosis, 3 cases of secondary loss of reduction and 5 cases of significant ectopic ossification. Conclusion: The outcome of these difficult fractures depends on restoration of hip joint congruity and stability and correlates closely to radiographic result

Purpose: The purpose of this work was to study arterial blood supply to the coxal bone in order to minimise the risk of postsurgical necrosis during acetabular osteosynthesis. Necrosis of the coxal bone is a rare but well-known complication of acetabular fracture surgery. Material and methods: Ten fresh cadavers were dissected after intra-arterial injection of coloured resin. All collaterals feeding the bone were described and counted. An arterial map was drawn. Results: The acetabulum is supplied by four main arterial sources: 1) the ischion artery, a collateral of the pudendal artery, which supplies the posterior and lateral part of the acetabulum; 2) the artery of the roof of the acetabulum, a collateral of the superior gluteal artery, supplies the upper and lateral part of the acetabulum: 3) branches of the anterior and posterior division of the obturator artery which supply the upper part and the rim of the obturated foramen and the anteroinferior and posteroinferior parts of the acetabulum; 4) branches issuing from the obturator artery supply the quadrilateral surface. Discussion: The Kocher approach can easily injure the ischion artery. The wide lateral approach described by Letournel and the triradiate approach described by Mears can injure the ischion artery and the artery of the roof of the acetabulum. Theoretically, the risk of bony necrosis would be greater if an endopelvic approach is associated due to the risk of injury to the endopelvic arteries issuing from the obturator artery. The anterior approach to the acetabulum appears to carry the less risk of ischemia, theoretically, than the other approaches to the acetabulum

This report reviews the long-term results of treating acetabula with unusually severe problems, such as pelvic discontinuity or major column loss after failed total hip arthroplasty (THA) and reconstruction problems. Loss of acetabular bone stock results from removal of bone during the original procedure, prosthetic failure, and osteolysis. In massive structural failure, the acetabular rim, quadrilateral plate, and associated columns become deficient. At worst, this may be combined with pelvic discontinuity and disruption of the ilium and ischium. Prosthetic protrusio may result from fixation loss and be associated with scarring of the femoral vessels, femoral nerve, ureter and bowel. A variety of implants has been used to in ace-tabular reconstruction. The results are often poor because of insufficient bone stock to support the implant. In a consecutive series of 251 THA revisions done between 1988 and 1996, 17 patients were treated for major pelvic column loss, pelvic discontinuity or both. In five patients, a posterolateral approach without trochanteric osteotomy was used. The extensile triradiate approach with ilioinguinal extension was used in 12 patients in whom severe prosthetic protrusio increased the risk of intrapelvic iatrogenic injury. A long anterior column pelvic plate was applied. A posteriorly placed AO 4.5-mm pelvic reconstruction plate with 10 to 12 holes was used in nine cases of pelvic discontinuity and in five cases of posterior column bone loss. This plate extended from the most inferior extent of the ischium across the wall of the posterior column to a point high on the ilium. Anterior column fixation was done in eight of nine cases of pelvic discontinuity and all three cases of anterior column deficiency. This called for an 8 to 12-hole 3.5-mm AO pelvic reconstruction plate that extended from the pubic symphysis across the pelvic rim. This spanned the anterior column defect, ranging from 4 cm to 8 cm, to the medial wall of the ilium. Bulk allograft was used in 16 of the 17 patients. The patient in whom allograft was not used had pelvic discontinuity following pelvic irradiation. Whole pelvic acetabular transplants were used in seven with severe bone loss or following resection for chondrosarcoma and the other for pigmented or villonodular synovitis. Posterior segmental acetabular allograft was used in two cases of posterior column absence. Femoral heads were used in two posterior column defects, three pelvic discontinuities with anterior column defect, and two anterior column defects. Acetabular components were cemented in six of seven whole bulk ace-tabular transplants, six of nine pelvic discontinuities and two anterior column defects. Cemented implants were classified as loose if there was a complete radiolucent line at the bone cement interface, measurable component migration or measurable change in position. Uncemented acetabular components were considered loose if component migration had occurred or screws had broken. Pelvic plates were considered loose if there was measurable migration or change in plate position or if fixation screws had backed out or broken. Radiographic union was considered present when bridging callus or trabecular bone was visible across the discontinuity site. Junctional healing was considered probable when radiographs did not show obvious signs of failure. Grafts were considered unhealed if there was obvious displacement, bone gaps or hardware breakage. Seven of the nine patients with pelvic discontinuity had late evidence of healing of the fracture and allograft consolidation. One underwent removal of the graft at three weeks after developing acute postoperative infection: early junctional healing of a whole bulk acetabular allograft required an osteotomy to break up the interface. Another patient, who underwent removal of the graft and implant at three months for chronic infection, had consolidation of a whole bulk ace-tabular allograft. One patient underwent revision of a pressfitted acetabular component at 60 months, and the pelvic discontinuity was solidly united. In a fourth patient, explored at 124 months for loosening of a cemented cup, there was near complete dissolution of the graft posterior acetabular wall and a loose posterior pelvic plate. In a patient with pelvic discontinuity after radiation therapy for uterine carcinoma, satisfactory healing of the pelvic discontinuity was confirmed at 32 months, when excisional arthroplasty for late chronic infection followed urinary sepsis. Seven patients had major column loss with severe cavitary defects. Consolidation of the allograft was noted in all seven within the first 12 months of follow-up. Revision (47%) was required for infection in three patients, implant loosening in four, and recurrent implant dislocation in one. The four loose cups were revised to a cemented all-polyethylene component. All four implants had been placed on less than 50% host bone. None of the four has required subsequent revision. Dislocation postoperatively occurred in eight patients. In six, the extensile triradiate approach had been used. This approach led to dislocation in 50%. The main reasons for using the extensile triradiate approach were to avoid catastrophic injuries by direct exposure of vital structures and to allow stable anterior column plate fixation. In that no neurovascular injuries occurred and stable durable allograft consolidation and healing of pelvic discontinuity took place, these goals were largely met. Three patients developed late sciatic palsy. In one, plaster immobilisation had possibly caused direct pressure over the fibular head and led to chronic peroneal palsy. The other two underwent additional exploration of the sciatic nerve for late entrapment caused by migration of screws from the posterior column plate. Two patients developed bladder infections postoperatively. Another developed superficial phlebitis of the lower leg. Acetabular revision for loosening was necessary in three of seven cementless implants, while only two of 10 cemented implants failed. The acetabular component should be cemented into the allograft when more than 50% of the prosthetic interface is non-viable. Virtually all graft material, including dense cortical grafts, may ultimately fail if used for implant fixation. Patients should be told about the inevitable risks. However, techniques used led to stable healing of the pelvic discontinuity in most cases. Long pelvic plates that securely stabilise the pelvis and allografts carefully opposed to host bone may explain the relative success in this series

Purpose: Among 80 surgical treatments for acetabular fracture, the Dana Mears approach was used in 15. The purpose of this study was to analyse functional and radiological outcome of these fractures at a mean follow-up of 41 months. Material and methods: The AO classification was used for fractures of the acetabulum : 12 class B (80%) with five B1a2 five B2a1 and two B1a1, and three class C (20%). There was one deformed callus (B1a2) at 120 days Two patients had associated pelvic injuries, eight a hip dislocation, and two an initial sciatic palsy. There were also two osteochondral fractures of the femoral head. The Dana Mears approach was modified slightly in the anterior part passing in front of the tensor muscle to preserve innervation. The gluteal muscles were raised by trochanterotomy. The displacement, the head/ roof congruency and the head/acetabulum congruency were assessed according to the 1981 SOFCOT criteria on the initial x-rays (AP pelvis, oblique ala and obturator) and computed tomographies. The quality of the reduction was assessed with the Matta and Duquesnoy-Senegas criteria. Clinical results were assessed with the Postel Merle d’Aubigné (PMA) score. Results: Radiographically, there was an anatomic reduction in 73.3% of the cases and perfect head/roof congruency in 80%. Functional outcome was excellent or good in 80% of the patients. Postoperative complications included 11 ossifications, and one transient sciatic paralysis. There was one late aseptic osteonecrosis of the femoral head. Discussion: The functional prognosis of these fractures is significantly correlated with the quality of reduction (p < 0.05). The advantage of this approach is the direct access to the roof without disinsertion of the gluteal muscles from the iliac crest, allowing more rapid recovery (seven to eight months) of medius gluteus function. In principal drawback is the very high rate of ossifications (one patient required revision for arthrolysis). Conclusion: The Dana Mears triradiate approach is an integral part of the surgical treatment of acetabular fractures, particularly for B1a2 and B2a1 fractures, but also for B1a1 transtectal fractures. Conversely, this approach is insufficient for reduction of type C fractures requiring and extensive access to the iliac wing and for surgery of deformed calluses where an endopelvic approach is indispensable to control the vessels

This paper reviews the causes of chronic instability after total hip arthroplasty (THA). The overall reported incidence varies from 0.5% to 9.5%. At 2% to 6%, the incidence following primary THA is higher with a posterior approach than with an anterior approach (0.5% to 3%). The incidence is reported to be as high as 22% after revision THA and 50% after extensile triradiate approach for pelvic discontinuity. Inadequate soft tissue lengthening, damaged abductors and nonunion of trochanteric osteotomy are known to predispose patients to chronic instability after THA. Elderly women are particularly susceptible. Poor patient compliance is also a cause. Surgical technique is also a factor. The lateral decubitus position often causes flattening of the lumbar lordosis, leading to potential cup retroversion. Over 90% of all dislocations are posterior, and disruption of external rotators and capsular damage should be repaired if possible. The optimal implant position appears to be 40° TO 45° of abduction, 15° to 20° of femoral anteversion, and 20° to 30° of cup flexion. Elevation of the hip centre weakens abductor pull, causing instability. Because a reduced femoral offset causes potential instability, this should be measured preoperatively to make sure that the stem can provide adequate offset. It may be necessary to add a thicker liner to increase the offset. Prosthetic factors which play a role in chronic instability include the use of smaller femoral heads, thick necked stems and heads with skirts. A larger femoral head increases stability simply by increasing the radian about the hip centre, increasing the potential range of motion. Extended posterior wall-adds improve the range of motion, and consequently the stability. However, there are fears that their use may increase the incidence of impingement and/or lead to increased wear. Skirted femoral heads impinge on the liner, limiting movement, and their use should be avoided in most cases of instability. Femoral stem offset relates to the neck shaft angle and the effective hip centre/shaft axis length or offset. It is easier to increase offset with lower neck shaft angle than to lengthen the leg. Because a bell curve is used in the design of femoral stems, many prosthetic systems lack adequate offset, especially when larger stems (48 mm to 52 mm) are used. In earlier prosthetic designs, bulk was added to the necks to eliminate stem breakage. In certain stems, the way in which dimensions were scaled meant the neck dimensions of larger prostheses were disproportionately big. We stopped using Depuy Stability stems sizes 16 mm and 18 mm because of this. Thornberry et al have shown that a circulotrapezoidal neck design is the best shape and leads to the least impingement. They have also shown that increasing the width of the chamfer of the acetabular liner rim improves the range of motion. In treating early instability (occurring less than 30 days postoperatively) most authors recommend bracing for six to eight weeks and warning patients severely about the long-term potential of redislocation. In cases of chronic instability (occurring more than 30 days postoperatively) all potential problems must be explored: these include soft tissue laxity, cup retroversion, inadequate offset, surgical approach, etc. In managing multiple dislocation, the use of extended immobilisation is less desirable although patients who have undergone revision have been subjected to a great deal of soft tissue dissection and potentially should be braced for up to 12 months. If the cause is correctable-malpositioning, soft tissue laxity or bony impingement – treatment is likely to be successful in 85% of cases. However, if the implants are in good position, the ‘bloodless revision’ (Fehring) has less than 50% chance of succeeding. The implication is that an extended posterior wall liner, longer modular femoral head, and soft tissue reconstruction are not going to work in the majority of cases. Designed by Noiles, the J& J SROM constrained acetabular liner uses a polyethylene capture mechanism that is secured by two additional screws. The pullout strength of this device is 1 350 N but torque required (lever-out strength) diminishes to 17.3 N.m for a 28-mm head. With a 32 mm head, 105° of flexion was obtained (while the normal hip needs up to 113° for usual flexion). Following up 21 patients with this implant for over two years, Anderson et al found redislocation in 29%. The only causative factor identified was an abduction angle of more than 70°. However, there were no cases of implant loosening of this device. Prevention of loosening was one of the design goals in using a ‘softer’ locking mechanism. Dislodgement of the liner requires immediate re-operation. The Osteonics constrained liner cup has a dual socket. The inner socket has a polished chrome surface manufactured fit to the outer socket. It fits a 22 mm or 28 mm head, and has a locking ring identical to the bipolar implant that holds the head in place. The implant can be snap-fitted into a 52-mm or larger Osteonics cup. This liner can also be cemented into another metal-backed liner. Goetz et al evaluated 56 cases, in 10 of which this implant had been cemented and in 46 lock-fitted in appropriately matched metal shells. In one case, the cemented constrained liner had separated from the metal shell. None of the constrained liners had separated from the metal shells, but one shell had loosened. There are many similar constrained acetabular liners. The choice is between a ‘locked’ liner that can never separate and a ‘softer’ lock that may protect fixation of the cup