Instability is the most common reason for revision after total hip arthroplasty (THA). Since THA requires arthrotomy of the hip and replacement with a femoral head that is smaller than the normal hip, instability following THA is always a potential concern. Many factors contribute to the development of instability after THA including: restoration of normal anatomy, implant design, component position, surgical approach and technique, and numerous patient related factors. Recently, the role of spinal mobility and deformity has been shown to have a significant effect on risk of dislocation after THA. The long held guidelines for component positioning or so called “safe zone” described by Lewinnek have also been questioned since most dislocations have been shown to occur in patients whose components are positioned within this “safe” range. In the early post-operative period, dislocation can occur prior to capsular and soft tissue healing if the patient exceeds their peri-operative range of motion limits. Closed reduction and abduction bracing for 6 weeks may allow for soft tissue healing and stabilization of the hip. It is important to try and identify the mechanism of dislocation since this can affect the technique of closed reduction, how the patient is braced following reduction and what may need to be addressed at the time of revision if dislocation recurs. Closed reduction and bracing may be effective in patients who have a previously well-functioning, stable THA who suffer a traumatic dislocation after the peri-operative period. Despite successful closed reduction, recurrent dislocation occurs in many patients and can be secondary to inadequate soft tissue healing, patient noncompliance or problems related to component positioning. Patients who incur more than 2 dislocations should be considered for revision surgery. Prior to revision surgery, an appropriate radiographic evaluation of the hip should be performed to identify any potential mechanical/kinematic issues that need to be addressed at the time of revision. Typically this involves plain radiographs, including a cross table lateral of the involved hip to assess acetabular version, but may also involve cross-sectional imaging to assess femoral version. Patients with soft tissue pseudotumors frequently have significant soft tissue deficiencies that are not amenable to component repositioning alone and require use of constrained or dual mobility components. In general, “limited revisions” consisting of modular head and liner exchange with insertion of a lipped liner and larger, longer femoral head rarely correct the problem of recurrent instability, since component malposition that frequently contributes to the instability is not addressed. Similarly, insertion of a constrained liner in a malpositioned cup is associated with a high rate of implant failure and recurrent dislocation since impingement contributing to the instability is not addressed. In patients who fail closed management and have a history of recurrent instability, we have found the treatment paradigm described by Wera, et al. to be very helpful in the management of the unstable THA. Several studies have shown that tripolar type constrained liners appear to perform considerably better than locking ring type constrained liners. As a result, dual mobility implants are becoming more widely utilised in patients with abductor and other soft tissue deficiencies, hip instability of uncertain etiology and patients with increased risk factors for instability undergoing primary THA. Early results with dual mobility components have been shown to have a low rate of failure in high instability risk revision THAs. These devices do have several unique potential complications and their use should be limited to patients with significantly increased risk of dislocation and instability

Introduction. Four parts inter trochanteric fracture of femur are commonest in elderly people. DHS fixation is gold standard treatment of such fractures. Various Complications of DHS implant are reported in the literature. However, Hip Instability: Subluxation and Dislocation is very rare. We report, five cases of Hip instability following DHS fixation surgery. Materials and Methods. This is a retrospective study conducted at ACPM Medical College, Dhule. We found only five cases that developed hip instability after DHS fixation since 1997. Available clinical notes and X-rays of these patients were studied to get the relevant information. Results and Observations. Three patients were male, two female, four had right sided fracture and one had left sided. Three had instability after six weeks and remaining two developed dislocation after eight months which were associated with infection. 1 patient refused further investigations & treatment, 2 other died with due course of time 1 lost to follow-up and one patient with deep infection underwent excision arthroplasty. We could only speculate cause for dislocation / subluxation on the basis of clinical examination, X-ray, Investigations and review of literature. In these cases it appeared that the factors responsible for instability could be mechanical factors and pyogenic infection. Review of literature and possible aetiological factors, investigations and various aspects of management of such cases are discussed. Conclusions. Mechanical factors such as intra-operative femoral head rotation, avulsion of greater trochanter, excessive medialisation, valgus reduction, excessive collapse, soft tissue injury, and infection are contributing factors for hip instability

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. At our center, we have tried to approach the unstable hip by identifying the primary cause of instability and correcting that at the time of revision surgery. Type 1:. Malposition of the acetabular component treated with revision of the acetabular component and upsizing the femoral head. Type 2:. Malposition of the femoral component treated with revision of the femur and upsizing the femoral head. Type 3:. Abductor deficiency treated with a constrained liner or dual mobility bearing. Type 4:. Soft tissue or bony impingement treated with removal of impingement sources and upsizing the femoral head. Type 5:. Late wear of the bearing treated with bearing surface exchange and upsizing the femoral head. Type 6:. Unclear etiology treated with a constrained liner or dual mobility articulation. These may be patients with abnormal spino-pelvic motion. The most common etiologies of instability in our experience include cup malposition (Type 1) and abductor deficiency (Type 3). We reviewed 75 hips revised for instability and at a mean 35.3 months 11 re-dislocations occurred (14.6%). Acetabular revisions were protective against re-dislocation (p<0.02). The number of previous operations (p=0.04) and previously failed constrained liners (p<0.02) were risk factors for failure. The highest risk of failure was in patients with abductor insufficiency with revisions for other etiologies having a success rate of 90%. Although instability can be multifactorial, by identifying the primary cause of instability, a rational approach to treatment can be formulated. In general the poorest results were seen in patients with abductor deficiency. Given the high rate of failure of constrained liners (9 of the 11 failures were constrained), we currently are exploring alternatives such as dual mobility articulations. Our early experience with dual mobility suggests improved results when compared to constrained liners

Hip instability is one of the early complications following total hip arthroplasty. The aetiology of hip instability is often multi-factorial. The aim of this study was to assess the efficacy of prosthesis with dual mobility system in reducing hip instability in high risk cases at a short term follow up. A retrospective analysis was performed covering 25 total hip arthroplasties on 24 patients (5 males and 19 females) between January 2007 and August 2008. Patient medical records and imaging were reviewed, and the indications for surgery and surgical outcome were analysed. Among 25 procedures performed, 18 were revision procedure for dislocations, loosening, peri-prosthetic fractures. Seven patients have primary procedure, among which two were for acute hip fractures. One patient underwent conversion from previously failed screw fixation for hip fracture. The 25 patients were followed up for a mean period of 12 months (ranging from 4 to 24 months). At the last follow up, all patients were able to mobilise pain-free either independently, or with aids; no patients presented an episode of hip instability. The result of our study demonstrated good early stability of total hip arthroplasty with dual mobility system (POLARCUP)

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. At our center, we have tried to approach the unstable hip by identifying the primary cause of instability and correcting that at the time of revision surgery. Type 1: Malposition of the acetabular component treated with revision of the acetabular component and upsizing the femoral head. Type 2: Malposition of the femoral component treated with revision of the femur and upsizing the femoral head. Type 3: Abductor deficiency treated with a constrained liner or dual mobility bearing. Type 4: Soft tissue or bony impingement treated with removal of impingement sources and upsizing the femoral head. Type 5: Late wear of the bearing treated with bearing surface exchange and upsizing the femoral head. Type 6: Unclear etiology treated with a constrained liner or dual mobility articulation. The most common etiologies of instability in our experience include cup malposition (Type 1) and abductor deficiency (Type 3). We reviewed 75 hips revised for instability and at a mean 35.3 months, 11 re-dislocations occurred (14.6%). Acetabular revisions were protective against re-dislocation (p<0.02). The number of previous operations (p=0.04) and previously failed constrained liners (p<0.02) were risk factors for failure. The highest risk of failure was in patients with abductor insufficiency with revisions for other etiologies having a success rate of 90%. Although instability can be multifactorial, by identifying the primary cause of instability, a rational approach to treatment can be formulated. In general, the poorest results were seen in patients with abductor deficiency. Given the high rate of failure of constrained liners (9 of the 11 failures were constrained), we currently are exploring alternatives such as dual mobility articulations. Our early experience with dual mobility suggests improved results when compared to constrained liners

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. At our center, we have tried to approach the unstable hip by identifying the primary cause of instability and correcting that at the time of revision surgery. Type 1: Malposition of the acetabular component treated with revision of the acetabular component and upsizing the femoral head. Type 2: Malposition of the femoral component treated with revision of the femur and upsizing the femoral head. Type 3: Abductor deficiency treated with a constrained liner or dual mobility bearing. Type 4: Soft tissue or bony impingement treated with removal of impingement sources and upsizing the femoral head. Type 5: Late wear of the bearing treated with bearing surface exchange and upsizing the femoral head. Type 6: Unclear etiology treated with a constrained liner or dual mobility articulation. The most common etiologies of instability in our experience include cup malposition (Type 1) and abductor deficiency (Type 3). We reviewed 75 hips revised for instability and at a mean 35.3 months 11 re-dislocations occurred (14.6%). Acetabular revisions were protective against re-dislocation (p<0.015). The number of previous operations (p=0.0379) and previously failed constrained liners (p<0.02) were risk factors for failure. The highest risk of failure was in patients with abductor insufficiency with revisions for other etiologies having a success rate of 90%. Although instability can be multifactorial, by identifying the primary cause of instability, a rational approach to treatment can be formulated. In general the poorest results were seen in patients with abductor deficiency. Given the high rate of failure of constrained liners (9 of the 11 failures were constrained), we currently are exploring alternatives such as dual mobility articulations

Background. Dislocation is a common complication after proximal and total femur prosthesis reconstruction for primary bone sarcoma patients. Expandable prosthesis in children puts an additional challenge due to the lengthening process. Hip stability is impaired due to multiple factors: Resection of the hip stabilizers as part of the sarcoma resection: forces acts on the hip during the lengthening; and mismatch of native growing acetabulum to the metal femoral head. Surgical solutions described in literature are various with reported low rates of success. Objective. Assess a novel 3D surgical planning technology by use of 3D models (computerized and physical), 3D planning, and Patient Specific Instruments (PSI) in supporting correction of young children suffering from hip instability after expandable prosthesis reconstruction following proximal femur resection. This innovative technology creates a new dimension of visualization and customization, and could improve understanding of this complex problem and facilitate the surgical decision making and procedure. Method. Two children, both patients with Ewing Sarcoma of the left proximal femur stage-IIB, ages 3/5 years at diagnosis, were treated with conventional chemotherapy followed by proximal femur resection. Both were reconstructed with expandable prosthesis (one at resection and other 4 years after resection). Hip migration developed gradually during lengthening process in the 24m follow up period. 3D software (Mimics, Materialise, Belgium) were used to make computerized 3D models of patients' pelvises. These were used to 3D print 1:1 physical models. Custom 3D planning software (MSk Lab, Imperial College London) allowed surgeons visualizing the anatomical status and assess of problem severity. Thereafter, osteotomies planes and the desired position of acetabular roof after reduction of hip joint were planned by the surgeons. These plans were used to generate 3D printed PSIs to guide the osteotomies during shelf and triple osteotomy surgeries. Accuracy of planning and PSIs were verified with fluoroscopy and post-op X-rays, by comparing cutting planes and post-op position of the acetabulum. Results. Surgeons reported excellent experience with the 3D models (computerized and physical). It helped them in the decision process with an improved understanding of the relationship between prosthesis head and acetabulum, a clear view of the osteophytes and bone formation surrounding the pseudoacetabulum, and osteophytes inside the native acetabulum. These osteophytes were not immediately visible on 2D CT imaging slices. Surgeons reported a good fit and PSIs' simplicity of use. The hip stability was satisfactory during surgery and in the immediate post-op period. X-ray showed a good and centered position of the hip and good levels of the osteotomies. Conclusions. 3D surgical planning and 3D printing was found to be very effective in assisting surgeons facing complex problems. In these particular cases neither CT nor MRI were able to visualize all bony formation and entrapment of prosthesis in the pseudoacetabulum. 3D visualisation can be very helpful for surgical treatment decisions, and by planning and executing surgery with the guidance of PSIs, surgeons can improve their surgical results. We believe that 3D technology and its advantages, can improve success rates of hip stability in this unique cohort of patients

Introduction:

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. The purpose of this study was to classify causes of instability and evaluate outcomes based on an algorithmic approach to treatment.

Introduction

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. The purpose of this study was to classify causes of instability and evaluate outcomes based on an algorithmic approach to treatment.

Introduction

Recurrent dislocation following total hip arthroplasty (THA) is a complex, multifactorial problem that has been shown to be the most common indication for revision THA. The purpose of this study was to classify causes of instability and evaluate outcomes based on an algorithmic approach to treatment.

We evaluated the use of a cemented Constrained Acetabular Component to treat recurrent or potential instability after hip replacement. Over a seven year period, 109 patients who had undergone 110 operations were identified from hospital records. Patients were reviewed based on clinical and radiological follow-up. Post-operative mobility and quality of life was assessed using the Oxford Hip Score (OHS) and SF-12.

From an original cohort of 109 patients, 9 patients were lost to follow up. Of the remaining 100 patients, the mean follow up was 2.9 years (SD+/−2 years). There were 4 failures, requiring 3 further revisions. The mean post-operative OHS was 33, SF-12 PCS 34 and SF12 MCS 52. 5-year survivorship was 90%. Cementing a Constrained Acetabular Component provides satisfactory mid term results in patients at high risk of hip dislocation.

Introduction

The published results of the use of a dual mobility cup to prevent instability in primary and revision total hip arthroplasty (THA) have established its efficacy. However, the monoblock, porous cobalt chromium cup design makes secure fixation difficult to achieve, limiting its use in patients with significant acetabular deformity or bone loss. Recently, a modular version of the dual mobility cup was introduced, consisting of a conventional porous shell with holes to allow augmented screw fixation, a highly polished modular metal liner, and a standard bipolar femoral head. The purpose of this report is to present its various indications, the surgical technique, and report our initial results.

Introduction. Previous research defines the existence of a “safe zone” (SZ) pertaining to acetabular cup implantation during total hip arthroplasty (THA). It is believed that if the cup is implanted at 40°±10° inclination and 15°±10° anteversion, risk of dislocation is reduced. However, recent studies have documented that even when the acetabular cup is placed within the SZ, high incidence dislocation and instability remains due to the combination of patient-specific configuration, cup diameter, head size, and surgical approach. The SZ only investigates the angular orientation of the cup, ignoring translational location. Translational location of the cup can cause a mismatch between anatomical hip center and implanted cup center, which has not been widely explored. Objective. The objective of this study is to define a zone within which the implanted joint center can be altered with respect to the anatomical joint center but will not increase the likelihood of post-operative hip separation or dislocation. Methods. A theoretical forward solution hip model, previously validated by telemetric devices and fluoroscopy data of existing implants, was used for analysis. The model allows for modifications of implant geometries/placement and soft tissue resection to simulate various surgical conditions. For the baseline simulation, the cup center was matched to the anatomical hip joint center, calculated as the center of the best fit sphere mapping the acetabulum, and the orientation of the cup was 40°/15° (inclination/anteversion). Keeping cup orientation the same, the location of the cup was moved in 1 mm increments in all directions to identify the region where a mismatch between the two centers did not lead to separation or instability in the joint. Results. During both swing and stance phase, when the acetabular cup was placed within the optimal conic with a slant height of 5±1 mm, no hip instability or dislocation risk occurred. As the acetabular cup was translated to the boundary of the optimal conic, hip instability increased. When the acetabular cup was placed at the boundary of the optimal conic, up to 2 mm of hip separation in the lateral direction occurred during swing phase, resulting in a decrease in contact area and an increase in contact stress. As the cup was placed outside the optimal conic, severe edge loading and hip separation up to 3.5 mm occurred during swing phase. In general, this resulted in large increases in cup stress, resulting in increased risk of wear leading to early complications. Discussion. This study introduces the concept of an optimal conic in the hip joint space to reduce the incidence of dislocation and hip instability after THA. Placing the cup center within the optimal conic reduces hip instability. Moving the cup further from the anatomical hip center increases the occurrence of hip instability. Cup placement within the optimal conic and angular SZ can lead to better postoperative outcomes. For any figures or tables, please contact authors directly

Introduction. Sagittal pelvic tilt (SPT) can change with spinal pathologies and fusion. Change in the SPT can result in impingement and hip instability. Our aim was to determine the magnitude of the SPT change for hip instability to test the hypothesis that the magnitude of SPT change for hip instability is less than 10° and it is not similar for different hip motions. Methods. Hip implant motions were simulated in standing, sitting, sit-to-stand, bending forward, squatting and pivoting in Matlab software. When prosthetic head and liner are parallel, femoral head dome (FHD) faces the center of the liner. FHD moves toward the edge of the liner with hip motions. The maximum distance between the FHD and the center in each motion was calculated and analyzed. To make the results more reliable and to consider the possibility of bony impingement, when the FHD approached 90% of the distance between the liner-center and liner-edge, we considered the hip “in danger for dislocation”. The implant orientations and SPT were modified by 1-degree increments and we used linear regression with receiver operating characteristic (ROC) curve and area under the curve (AUC) to determine the magnitude of SPT change that could cause instability. Results. SPT modification as low as 7° could result in dislocation during pivoting (AUC: 87.5; sensitivity: 87.9; specificity 79.8; p=0.0001). This was as low as 10° for squatting (AUC: 91.5; sensitivity: 100; specificity 75.9; p=0.0001) and as low as 13° for sit-to-stand (AUC: 94.6; sensitivity: 98; specificity 83; p=0.0001). SPT modification affects hip stability more in pivoting than sit-to-stand and squatting. Discussion. Our results show the importance of close collaboration between the hip and spine surgeons in treating patients who undergo THA and spinal fusion. The postoperative SPT modification should be considered for preoperative computer simulation for determining the implant safe zone

Dynamic 2D sonography of the infant hip is a commonly used clinical procedure for developmental dysplasia of the hip (DDH) screening. It however has been found to be unreliable with some studies reporting associated misdiagnosis rates of up to 29%. In a recent systematic review, Charlton et al. examined dynamic ultrasound (US) screening for hip instability in the first six weeks after birth and found current best practices for such early screening techniques to be divergent between international institutions in terms of clinical scanning protocols. Such protocols include: the appropriate scanning plane and US probe position (e.g. coronal, transverse, lateral, anterior), DDH diagnostic metrics (e.g. femoral head coverage, alpha angle), appropriate patient age when scanning, and follow up procedures. To improve reliability of diagnosis and to help in standardizing diagnosis across different raters and health-centers, we propose an automated method for dynamically assessing hip instability using 3D US. 38 infant hips from 19 patients were scanned with B-mode 3D US by a paediatric orthopaedic surgeon and two technologists from the radiology department at a paediatric tertiary care centre. To quantify hip assessment, we proposed the use of femoral head coverage variability (ΔFHC3D) within 3D US volumes collected during a sequence of US scans (one at rest, and another with posterior stress applied to the joint as maneuvered during a dynamic assessment). We used phase symmetry image features to localize the ilium's vertical cortex and a random forest classifier to identify the location of the femoral head. The proposed ΔFHC3D provided good repeatability with an average test-retest ICC measure of 0.70 (95% confidence interval: 0.35 to 0.87, F(21,21) = 7.738, p<.001). The mean difference of ΔFHC3D measurements was 0.61% with a SD of 4.05%. Since the observed changes in ΔFHC3D start near 0% and range up to about 18% from stable to mildly unstable hips in this cohort, the mean difference and standard deviation of ΔFHC3D measurements observed suggest that the proposed metric and technique likely have sufficient resolution and repeatability to quantify differences in hip laxity. The long-term significance of this approach to evaluating dynamic assessments may lie in increasing early diagnostic accuracy in order to prevent dysplasia remaining undetected prior to manifesting itself in early adulthood joint disease

Complex acetabular reconstruction for oncology and bone loss are challenging for surgeons due to their often hostile biological and mechanical environments. Titrating concentrations of silver ions on implants and alternative modes of delivery allow surgeons to exploit anti-infective properties without compromising bone on growth and thus providing a long-term stable fixation. We present a case series of 12 custom acetabular tri-flange and custom hemipelvis reconstructions (Ossis, Christchurch, New Zealand), with an ultrathin plasma coating of silver particles embedded between layers of siloxane (BioGate HyProtect™, Nuremberg, Germany). At the time of reporting no implant has been revised and no patient has required a hospital admission or debridement for a deep surgical site infection. Routine follow up x-rays were reviewed and found 2 cases with loosening, both at their respective anterior fixation. Radiographs of both cases show remodelling at the ilium indicative of stable fixation posteriorly. Both patients remain asymptomatic. 3 patients were readmitted for dislocations, 1 of whom had 5 dislocations within 3 weeks post-operatively and was immobilised in an abduction brace to address a lack of muscle tone and has not had a revision of their components. Utilising navigation with meticulous implant design and construction; augmented with an ultrathin plasma coating of silver particles embedded between layers of siloxane with controlled and long-term generation of silver ion diffusion has led to outstanding outcomes in this series of 12 custom acetabular and hemipelvis reconstructions. No patients were revised for infection and no patients show signs of failure of bone on growth and incorporation. Hip instability remains a problem in these challenging mechanical environments and we continue to reassess our approach to this multifaceted problem

Hip instability is one of the most common causes for total hip arthroplasty (THA) revision surgery. Studies have indicated that lumbar fusion (LF) surgery is a risk factor for hip dislocation. Instrumented spine fusion surgery decreases pelvic tilt, which might lead to an increase in hip motion to accommodate this postural change. To the best of our knowledge, spine-pelvis-hip kinematics during a dynamic activity in patients that previously had both a THA and LF have not been investigated. Furthermore, patients with a combined THA and LF tend to have greater disability. The purpose was to examine spine-pelvis-hip kinematics during a sit to stand task in patients that have had both THA and LF surgeries and compare it to a group of patients that had a THA with no history of spine surgery. The secondary purpose was to compare pain, physical function, and disability between these patients. This cross-sectional study recruited participants that had a combined THA and LF (n=10; 6 females, mean age 73 y) or had a THA only (n=11; 6 females, mean age 72 y). Spine, pelvis, and hip angles were measured using a TrakSTAR motion capture system sampled at 200 Hz. Sensors were mounted over the lateral thighs, base of the sacrum, and the spinous process of the third lumbar,12th thoracic, and ninth thoracic vertebrae. Participants completed 10 trials of a standardized sit-to-stand-to-sit task. Hip, pelvis, lower lumbar, upper lumbar, and lower thoracic sagittal joint angle range of motion (ROM) were calculated over the entire task. In addition, pain, physical function, and disability were measured with clinical outcomes: Hip Disability Osteoarthritis Outcome Score (pain and physical function), Oswestry Low Back Disability Questionnaire (disability), and Harris Hip Score (pain, physical function, motion). Physical function performance was measured using 6-Minute Walk Test, Stair Climb Test, and 30s Chair Test. Angle ROMs during the sit-to-stand-to-sit task and clinical outcomes were compared between THA+LF and THA groups using independent t-tests and effect sizes (d). The difference in hip ROM was approaching statistical significance (p=0.07). Specifically, the THA+LF group had less hip ROM during the sit-to-stand-to-sit task than the THA only group (mean difference=11.17, 95% confidence interval=-1.13 to 23.47), which represented a large effect size (d=0.83). There were no differences in ROM for pelvis (p=0.54, d=0.28) or spinal (p=0.14 to 0.97; d=0.02 to 0.65) angles between groups. The THA+LF group had worse clinical outcomes for all measures of pain, physical function, and disability (p=0.01 to 0.06), representing large effect sizes (d=0.89 to 2.70). Hip ROM was not greater in the THA+LF group, and thus this is unlikely a risk factor for hip dislocation during this specific sit-to-stand-to-sit task. Other functional tasks that demand greater excursions in the joints should be investigated. Furthermore, the lack of differences in spinal and pelvis ROM were likely due to the task and the THA+LF group had spinal fusions at different levels. Combined THA+LF results in worse clinical outcomes and additional rehabilitation is required for these patients

Larger diameter femoral heads and improved operative approaches and soft tissue repair/closure have somewhat reduced the incidence of recurrent instability after total hip arthroplasty (THA). Nevertheless, hip instability remains one of the most common reasons for reoperation after THA, and accounts for roughly a quarter of hip revisions in the United States in Medicare patients. The prevalence of instability after THA varies widely, from 0.3% to 15%. Surgeons have come to understand that hip instability can be caused by implant malposition, impingement, and inadequate soft tissue tension or integrity. While the cumulative risk of instability is acceptable at approximately 2.8% with transtrochanteric approaches, this is based upon the trochanter actually healing (and often being advanced). On the other hand, trochanteric nonunion and proximal migration have been noted by many, and this frequently results in catastrophic instability. Moreover, and importantly, abductor insufficiency is one the most difficult causes of hip instability to solve. Woo and Morrey reported a 17.6% instability rate when trochanteric nonunion occurred with 1 cm proximal trochanteric migration. Alternatively, the contemporary incidence of instability with the posterolateral or anterolateral approaches, and an adequate soft tissue repair, is approximately 1–2%

Introduction. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. Methods. Cup orientation in 50 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 184 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Supine tilt-adjusted Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p< .0001). Supine tilt-adjusted Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p<.01). Alt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 30.6 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 +/− 7.8 degrees of tilt-adjusted operative anteversion and 42.4 +/− 13.5 degrees of operative inclination (Figure 1). Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup mal-position and its associated complications. For figures and tables, please contact authors directly

Introduction. Cup malpositioning remains a common cause of dislocation, wear, osteolysis, and revision. The concept of a “Safe Zone” for acetabular component orientation was introduced more than 35 years ago1. The current study assesses CT studies of replaced hips to assess the concept of a safe zone for acetabular orientation by comparing the orientation of acetabular components revised due to recurrent instability and to a series of stable hip replacements. Methods. Cup orientation in 21 hips revised for recurrent instability was measured using CT. These hips were compared to a group of 115 stable hips measured using the same methods. Femoral anteversion in the stable hips was also measured. Images to assess femoral anteversion in the unstable group were not available. An application specific software modules was developed to measure cup orientation using CT (HipSextant Research Application 1.0.13 Surgical Planning Associates Inc., Boston, Massachusetts). The cup orientation was determined by first identifying Anterior Pelvic Plane Coordinate system landmarks on a 3D surface model. A multiplanar reconstruction module then allowed for the creation of a plane parallel with the opening plane of the acetabulum. The orientation of the cup opening plane in the AP Plane coordinate space was calculated according to Murray's definitions of operative anteversion and operative inclination2. Both absolute cup position relative to the APP and tilt-adjusted cup position3 were calculated. Results. Operative anteversion for the anteriorly unstable hips was significantly higher than in the stable hips (p < .001). Operative anteversion for the posteriorly unstable hips was significantly lower than in the stable hips (p=.01). Adjusting for pelvic tilt in the supine position, all unstable hips had operative anteversion of less than 22.9 or more than 38.6 degrees or operative inclination of less than 28.9 or more than 55.9 degrees or both. The center of the “safe zone” is 30.7 +/− 7.8 degrees of tilt-adjusted operative anteversion and 42.4 +/− 13.5 degrees of operative inclination. Conclusions. The current study demonstrates that most conventionally placed acetabular components are malpositioned but not all malpositioned acetabular components are associated with dislocation. Using acetabular revision for recurrent instability as the end point, a safe zone for acetabular component orientation does exist. The range is narrower for anteversion than for inclination. Improved methods of defining component positioning goals on a patient-specific basis and accurately placing the acetabular component may reduce the incidence of cup malposition and its associated complications