Aims. The aim of this study was to evaluate the correlation between Salter’s criteria and Kalamchi’s classification of avascular necrosis in patients treated for developmental dysphasia of the hip (DDH). Patients and Methods. The study involved a retrospective analysis of 123 patients (123 hips) with DDH treated by operative and non-operative reduction before the age of two years, with a minimum follow-up of ten years. Salter’s criteria (S1 to S4) were determined from radiographs obtained at one to two years post-reduction, whilst the Kalamchi grade was determined from radiographs obtained at ten or more years of age. Early post-reduction radiographs were also used to evaluate the centre-head distance discrepancy (CHDD) and the occurrence of a dome-shaped deformity of the proximal femoral metaphysis (D-shaped metaphysis). The prognosis was described as good (Kalamchi grade K0 or KI), fair (Kalamchi grade KII) or poor (Kalamchi grade KIII or KIV) for analysis and correlation with the early Salter criteria, CHDD and D-shaped metaphysis. Results. S1 and S2 criteria were predictive of a poor prognosis. The outcome following S3, S4 and S3 + S4 varied; 18 (40%) had a good prognosis, 17 (38%) a fair prognosis and ten (22%) a poor prognosis. A CHDD ≥ 10% and a D-shaped metaphysis were also predictive of a poor prognosis. Conclusion. The Salter criteria were predictive of the Kalamchi grade of avascular necrosis in patients with DDH aged ten or more years after reduction of the hip. . Cite this article: Bone Joint J 2017;99-B:1115–20

A special surgical technique and consideration is necessary in the total hip arthroplasty for dysplastic osteoarthritis after Kalamchi and MacEwen Type III or IV deformity (so called “Perthes-like-deformity”). There have been few reports concerning the total hip arthroplasty for “Perthes-like-deformity”. We evaluated the clinical and radiological outcome of 52 uncemented hip arthroplasties for the lesion. We have performed 106 hips of uncemented total hip arthroplasty for dysplastic osteoarthritis after Kalamchi and MacEwen Type III or IV deformity. Among them, 52 hips of 47 patients (11 males and 41 females) were evaluated with minimum of three years follow-up. The average age at the surgery was 52 (28 to 65). The average follow-up period was 4.8 (3 to 8.1) years. Against the developmental dysplasia or dislocation, 29 hips of 26 patients had been treated by casting or surgery in infancy. Thirteen hips of 11 patients had no previous treatment before the arthroplasty. Spongiosa metal cup (GHE: ESKA implants, Lübeck, Germany) was used for 33 hips of 28 patients and Zweymüller type cup (Allo-classic cup: Zimmer Inc., Warsaw, IN, Bicon cup: Smith & Nephew Orthopedics AG, Rotkreuz, Switzerland) for 19 hips of 19 patients. Spongiosa Metal stem (GHE: ESKA implants) was used for 23 hips of 19 patients and Zweymüller type stem (Alloclassic stem: Zimmer Inc., SL stem: Smith & Nephew Orthopedics AG) for 29 hips of 28 patients. The average operative time was 108 (53 to 233) minutes. The average blood loss during the surgery was 731(150 to 1749) milliliters. The adductor tendon release was added in 28 hips of 26 patients against the severe contracture. The patients were evaluated clinically (pre-surgical history, hip score, leg length discrepancy, Trendelenburg sign, and gait function) and radiologically (ATD before the surgery, alignment, and stability of implants). Average ATD before the surgery was −2.2 (−28 to 17) millimeters. The average leg length discrepancy was 1.9 (0 to 7) centimeters before the surgery and was improved to 0.1 (0 to 1) centimeters after the surgery. The average hip score was 54 (23 to 80) before the surgery and was improved to 90 (69 to 100) after the surgery. At the final follow-up, Trendelenburg sign was positive in 14 hips of 14 patients (26.9%) and the limping was not obvious in 38 hips of 33 patients (73.1%). All implants were stable at the final follow-up. “Perthes-like-deformity” often has the severe deformity. It has a shortening or an absence of the neck and an excessive antetorsion of the femur. When it has the coxa magna, the acetabulum is shallow, has the narrow anteroposterior diameter, and has the thin wall like the osteophyte. It is frequently accompanied by shortening of leg and contracture, as the lesion arises from the development disorders. Thus, the total hip arthroplasty, especially uncemented one, is complicated. However, the satisfactory result can be obtained by careful consideration and surgical procedure such as a provision against the bleeding and the soft tissue release

Aims. The aim of this study was to compare outcomes of guided growth and varus osteotomy in treating Kalamchi type II avascular necrosis (AVN) after open reduction and Pemberton acetabuloplasty for developmental dysplasia of the hip (DDH). Methods. This retrospective study reviewed patients undergoing guided growth or varus osteotomy for Kalamchi type II AVN between September 2009 and January 2019. All children who had undergone open reduction and Pemberton acetabuloplasty for DDH with a minimum two-year follow-up were enrolled in the study. Demographic and radiological data, which included the head-shaft angle (HSA), neck-shaft angle (NSA), articulotrochanteric distance (ATD), Sharp angle (SA), and lateral centre-edge angle (LCEA) at baseline, two years, and at the extended follow-up, were compared. Revision rates were evaluated. Clinical outcomes using the Harris Hip Score were assessed two years postoperatively. Results. A total of 24 patients underwent guided growth and 19 underwent varus osteotomy, over a mean period of 3.3 years (95% confidence interval (CI) 2.8 to 3.8) and 5.2 years (95% CI 4.5 to 6.0), respectively. There were no differences in demographic and preoperative radiological data, except for a younger age at time of acetabuloplasty and larger ATD for the osteotomy group. The HSA did not differ at two years and the extended follow-up because of postoperative rebound in the osteotomy group. The NSA of the osteotomy group remained smaller postoperatively. There were no significant differences in the follow-up ATD, SA, and LCEA, except for a smaller two-year ATD of the osteotomy group. Seven patients (29.2%) in the guided growth group underwent revision surgery and none in the osteotomy group. The Harris Hip Score was similar between groups. Conclusion. Guided growth and varus osteotomy had comparable results in improving caput valgum deformity, given the rebound of lateral tilting of the physis after osteotomy correction. There were no differences in clinical outcomes at two years postoperatively. Cite this article: Bone Joint J 2022;104-B(7):902–908

Aims. To determine the likelihood of achieving a successful closed reduction (CR) of a dislocated hip in developmental dysplasia of the hip (DDH) after failed Pavlik harness treatment We report the rate of avascular necrosis (AVN) and the need for further surgical procedures. Methods. Data was obtained from the Northern Ireland DDH database. All children who underwent an attempted closed reduction between 2011 and 2016 were identified. Children with a dislocated hip that failed Pavlik harness treatment were included in the study. Successful closed reduction was defined as a hip that reduced in theatre and remained reduced. Most recent imaging was assessed for the presence of AVN using the Kalamchi and MacEwen classification. Results. There were 644 dislocated hips in 543 patients initially treated in Pavlik harness. In all, 67 hips failed Pavlik harness treatment and proceeded to arthrogram (CR) under general anaesthetic at an average age of 180 days. The number of hips that were deemed reduced in theatre was 46 of the 67 (69%). A total of 11 hips re-dislocated and underwent open reduction, giving a true successful CR rate of 52%. For the total cohort of 67 hips that went to theatre for arthrogram and attempted CR, five (7%) developed clinically significant AVN at an average follow-up of four years and one month, while none of the 35 hips whose reduction was truly successful developed clinically significant AVN. Conclusion. The likelihood of a successful closed reduction of a dislocated hip in the Northern Ireland population, which has failed Pavlik harness treatment, is 52% with a clinically significant AVN rate of 7%. As such, we continue to advocate closed reduction under general anaesthetic for the hip that has failed Pavlik harness. Cite this article: Bone Jt Open 2021;2(8):594–598

The late results of treatment of 24 legs in 21 children with congenital deficiency of the tibia are presented. A new classification is proposed which correlates well with recommendations for treatment and with the final functional result. Three types of deficiency were recognised: Type I, total absence of the tibia; Type II, distal absence; Type III, distal deficiency with tibiofibular diastasis. The early radiographic appearances, the functional status of the quadriceps and the severity of flexion contracture of the knee were important factors in the selection of the operations likely to give the best function.

Ninety-seven limbs, in eighty-one patients, with a diagnosis of congenital deficiency of the fibula have been reviewed. A classification was devised to distinguish the minimal hypoplasia of the fibula (Type I) from the well-known complete absence (Type II). Congenital anomalies of the femur were present in 76 per cent of patients with Type I deficiency and in 59 per cent with Type II. The shortening of the limb was by 13 per cent in Type I and by 19 per cent in Type II, and the percentage shortening was fairly constant during growth. A detailed description of the spectrum of other congenital anomalies was found to be characteristic: for example, the ball and socket formation of the ankle, tarsal coalition and anomalies of the foot. The treatment aimed simply to equalise leg length in Type I deficiency, while amputation of the foot and the fitting of a prosthesis were necessary in Type II to obtain satisfactory function.

A simple modification of Gallie's subtalar fusion is described as a salvage procedure in treating patients with pain from old fractures of the calcaneous involving the subtalar joint. Graft bone for the fusion is taken from the outer half of the calcaneus, thus avoiding disturbance of the tibia or iliac crest. Collapse of the donor site helps to narrow the widened heel present in these patients. The posterior approach allows the peroneal tendons to be freed from any adhesions, and at the same time release of the calcaneo-fibular ligament permits some correction of the valgus of the heel. The early results in six patients have been encouraging.

Introduction and Aims: To review the existing classifications in characterising the pathologic morphology of congenital lower limb deficiencies (CLLLD) and their usefulness in planning limb reconstruction. Method: Ninety-five patients undergoing limb reconstruction were classified using existing classifications. Predominantly femoral deficiencies were classified using Aitken, Amstutz, Hamanishi, Gillespie and Torode, Fixsen and Lloyd-Roberts, Kalamchi, and Pappas systems and fibular deficiencies were classified using Coventry and Johnston, Achterman and Kalamchi, and Birch systems. Results: All patients with predominant deficiency of one segment (femoral or fibular) also had associated shortening of the other segment in the same limb. Acetabular dysplasia, knee instability due to cruciate insufficiency and lateral femoral condylar hypoplasia were found in both femoral and fibular deficiencies. None of the existing classification systems were able to represent the complete pathologic morphology in any given patient. Due consideration of alignment, joint stability and length discrepancy of affected limb as a whole at the planning stage of reconstruction could not be ascertained using these classification systems. Instead, it was useful to characterise the morphology of the involved limb using the following method:. Acetabulum: Dysplastic/Non-dysplastic. Ball (Head of femur): Present/Absent. Cervix (Neck of femur): Pseudoarthrosis and neck-shaft angle. Diaphysis of femur: Length/deformity. Knee: Cruciates. Fibula and Tibia: Length/deformity. Ankle: Normal/Ball and socket/valgus. Heel: Tarsal coalition/deformity. Ray: Number of rays in the foot. Conclusion: Existing classifications do not represent the complete morphology of the entire involved lower limb in CLLLD and therefore a systematic method of characterising the morphology of the lower limb is more useful in planning limb reconstruction

Objective: To review the existing classifications in characterizing the pathological morphology of congenital lower limb deficiencies and their usefulness in planning limb reconstruction. Methods: Ninety-five patients undergoing limb reconstruction were classified using existing classifications. Predominantly femoral deficiencies were classified using Aitken,Amstutz,Hamanishi,Gillespie andTorode,Fixsen and Lloyd-Roberts, Kalamchi, and Pappas systems and fibular deficiencies were classified using Coventry and Johnston, Achterman and Kalamchi, and Birch systems. Results: All patients with predominantly femoral deficiencies also had associated shortening of ipsilateral tibia and fibula. Similarly, most patients with predominantly fibular deficiencies also had some associated shortening ipsilateral femur. Acetabular dysplasia, knee instability due to cruciate insufficiency and lateral femoral condylar hypoplasia were found in both femoral and fibular deficiencies. None of the existing classification systems were able to represent the complete pathologic morphology in any given patient. Due consideration of alignment, joint stability and length discrepancy of affected limb as a whole at the planning stage of reconstruction could not be ascertained using these classification systems. Instead, it was useful to characterize the morphology of the involved limb using the following method:. Acetabulum: Dysplastic/ Non-dysplastic (AC index, Sharp’s angle, CE angle). Ball (Head of femur): Present/Absent. Cervix (Neck of femur): Presence of pseudoarthrosis & neck-shaft angle. Diaphysis of femur: Length / deformity. Knee: Presence of cruciates, patellar and femoral con-dylar hypoplasia. Fibula and Tibia: Presence/ absence, length and deformity. Ankle: Normal/Ball and socket/ valgus. Heel: Presence of tarsal coalition and deformity (valgus, equinus). Ray: Number of rays present in the foot. Conclusion: Congenital longitudinal lower limb deficiency is a spectrum of disorder involving the entire lower limb. Existing classifications do not represent the complete morphology of the entire involved lower limb and therefore a systematic method of characterizing the morphology of the lower limb is more useful in planning limb reconstruction

The treatment of developmental dysplasia of the hip (DDH) in children remains controversial, we describe the clinical and radiological outcomes of 47 hips in 43 children treated with open surgery by one surgeon between 2004 and 2008 for DDH. The mean age at operation was 25 months (5 to 113) with a mean follow up of 89 months (22 to 169). 46 hips had an anterior open reduction, 1 had a medial approach performed and 16 had anterior open reductions only. 5 of the primary operations also had a pelvic osteotomy, 7 had a femoral osteotomy and 18 had a combined femoral and pelvic osteotomy. 7 (15%) of the hips required a second operation for dislocation, subluxation or dysplasia. At the latest follow up 40 of the 45 hips where Severin grades were recordable (89%) were graded as excellent or good, Severin class I or II. Clinically significant AVN (grade II to III according to the Kalamchi and MacEwen classification) was seen in 5 (11%) of the hips. We found a pelvic osteotomy to be a risk factor for AVN (p 0.02) and age at operation to be a risk factor for poor morphology at final follow up (p 0.03). We proceed to open surgery in patients over 12 months old or those with failed closed reduction. Over 18 months old a pelvic osteotomy should be performed in selective cases depending on intra-operative stability, but we will now consider doing this as a staged procedure and delaying the osteotomy for a period of time after open reduction to reduce the risk of AVN. We will also have a much lower threshold for performing a femoral shortening osteotomy in these patients as open reduction with Salters osteotomy alone tended to have a poorer outcome

The long term results of closed reduction of the hip for DDH were reviewed to determine if the presence of the ossific nucleus had an effect on outcome. The clinical and radiological outcome of a single-surgeon series of closed reduction for DDH was assessed in a strictly defined group of 48 hips in 42 patients with an average of 11.1 years follow up. In 50% of cases, the ossific nucleus was absent. 100% of patients had an excellent or good result (Severin classification) at final follow-up. 8.3% (4 hips) demonstrated evidence of avascular necrosis. Three were Kalamchi & MacEwen Type I and one was type II. Two of the AVN cases did not demonstrate an ossific nucleus at closed reduction, and both developed type I AVN. 6 hips underwent further surgery. The acetabular index and center-edge angle were not significantly different between the affected and unaffected hip at final follow-up. There was no relationship between the presence or absence of an ossific nucleus at the time of closed reduction and the final outcome. In this well defined group, closed reduction is safe and provides excellent results in the long-term. The absence of an ossific nucleus is not detrimental to the final outcome

Tibial dysplasia is a rare congenital deformity which must be distinguished from the more common fibular dysplasia. We have reviewed 24 patients with 35 affected legs. The classification system of Kalamchi and Dawe (1985) was found to be preferable to that of Jones, Barnes and Lloyd-Roberts (1978) as a guide to prognosis and management. We discuss the outcome of surgical treatment, recommending selective amputation for most cases

The authors evaluate the incidence, patterns and causative factors of avascular necrosis (AVN) in patients with developmental dysplasia of the hip (DDH) and to follow up these patients to determine what their long term functional and radiological outcome is. All patients treated for DDH by the same consultant with the subsequent development of AVN were assessed. Outcome was assessed by grading the AVN using the Kalamchi and McEwan classification at final follow up. A group of 250 hips with DDH were treated over a 16 year period and reviewed. All hips that developed AVN were studied. AVN was seen in 15% of hips treated with closed reduction and 62% of hips after open reduction–32% of the hips treated in the open reduction group were treated elsewhere and subsequently referred. If use of a Pavlik harness fails, children with DDH should be treated with pre reduction traction, closed reduction and spica cast after the age of 4 months. In the surgical group a capsulorrhaphy should be avoided. Poor radiological outcome at final follow up was not necessarily equivalent to a poor clinical outcome

The results of the Ferguson medial approach for open reduction of developmental dysplasia of the hip (DDH) were reviewed for 49 hips with a follow-up of more than 48 months. The mean age at operation was 12.3 months (6 to 23). The mean length of clinical and radiological follow-up was 82 months (48 to 148). Three redislocations occurred. Group I avascular necrosis according to the classification of Kalamchi and MacEwen was seen in four hips, group II in two hips and group III in one hip; 92% of the hips were classified as Severin class I and II. The acetabular index and centre edge (CE) angles were within normal limits at final follow-up, but were still significantly different from the unaffected side. We conclude that the Ferguson procedure is safe and reliable for low dislocations in children aged six to 18 months

Purpose. To compare the early medial open approach (MO) with the anterior approach (AO) performed after the appearance of the ossific nucleus for DDH that has failed closed reduction or presented late. Methods. We present the experience of 2 UK surgeons with prospectively gathered data for MO (26 hips) compared with that of a third surgeon in the same unit for the AO (21 hips) in 41 children under 24 months of age at index surgery. Femoral head osteonecrosis (FHO) risk was predicted using the height-to-width index of Bruce et al, measured at 12–18 months post reduction, and graded with the Kalamchi and MacEwen classification where follow-up exceeded 3 years. Acetabular index (AI) was measured at or close to 2 years post reduction. Results. Age at time of surgery averaged 11.2 months (3.1–24) for the MO group and 17.8 months (12–24) for the AO group. Average follow-up was 4.3 years (13 months to 12 years). FHO was evident or strongly predicted in 2/26 hips (7.7%) in the MO group and 2/21 (9.5%) in the AO group. AI improved by 8.8° (4–12°) and 7.9° (6–10°) respectively at 2 years post reduction (p>0.05). One case of early recurrent dislocation in the MO group required revision surgery via an anterior approach. The height to width index cut-off value of 0.357 at 12–18 months post reduction accurately predicted FHO risk in cases with longer follow-up. Conclusions. The medial open approach was not associated with a higher risk of FHO compared to a protocol of waiting for the appearance of the ossific nucleus before proceeding to reduction via an anterior approach. There was no significant difference in acetabular remodeling in the first two postoperative years between the protocols despite earlier reduction in the MO group

The management of developmental dysplasia of the hip (DDH) requiring open reduction between 12 and 18 months of age is controversial. We compare the outcome of medial approach open reduction (MAOR) versus delayed anterior open reduction with Salter osteotomy in such patients. 17 consecutive patients who underwent MAOR aged 12–20 months were reviewed (mean follow-up of 40 months, range 6–74). This group was compared to 15 controls who underwent anterior reduction and Salter osteotomy aged 18–23 months (mean follow-up of 44 months, range 14–134). 13 of the 17 (76%) MAOR patients required subsequent Salter osteotomy at a mean of 22 months post-reduction, with a further 2 patients under follow-up being likely to require one. Acetabular index improved from 42 (32–50, SD − 5.5) to 16 (7–24, SD − 4.5) in the MOAR group after Salter osteotomy compared to an improvement of 40 (30–53, SD − 6) to 13 (4–24, SD − 5) in the control group (p>0.05). Acetabular index at last follow-up was within normal limits in 15 of 17 (88%) MAOR patients. All patients in the control group had acetabular indices (or centre-edge angles of Wiberg) within the normal range. There was 1 subluxation (7%) in the control group. There were 6 cases (33%) of post-operative avascular necrosis (5 Kalamchi & MacEwen Grade I, 1 Grade 2) in the MAOR group and 6 (40%) in the control group (5 Grade 1, 1 Grade 4). All of the MAOR patients had good or excellent clinical results according to McKay's criteria, compared to 14 out of 15 (93%) controls. This study suggests that MAOR or delayed open reduction and Salter osteotomy is a reasonable treatment for children with DDH presenting between the ages of 12 and 18 months. However, the majority of MAORs are likely to require a subsequent Salter osteotomy

We reviewed 33 patients (35 hips) after open reduction of congenital dislocation of the hip using Ludloff’s medial approach. The mean age at the time of operation was 14 months (5 to 29) and at the time of final follow-up 20.1 years (15 to 24) giving a mean duration of follow-up of 19.4 years (14 to 23). We evaluated the radiological results by the Severin classification and the extent of avascular necrosis using the criteria of Kalamchi. At the latest review 16 hips (45.7%) were in Severin groups I or II (acceptable results) and 19 (54.3%) in Severin groups III, IV or V (unacceptable results). There was evidence of avascular necrosis in 15 hips (42.9%). The radiological results began to become worse at about the age of ten years when type-II avascular changes presented. We conclude that Ludloff’s medial approach for open reduction is unsatisfactory for the treatment of congenital dislocation since about half of our patients required additional operations

Ischemic necrosis of the femoral head occurring after the treatment of congenital dysplasia of the hip can negatively affect the long-term prognosis of the involved hip. The purpose of the study was to evaluate a number of clinical and radiological risk factors for AVN after non-operative treatment of DDH. Clinical data and radiographs of 77 patients with103 abnormal hips treated because of developmental dysplasia of the hip by closed reduction followed by cast immobilization were reviewed retrospectively. The average age of patients at the time of reduction was 16 months (ranged, 4 to 28) and the average final follow up was 22,4 years (ranged from 13 to 47 years). Kalamchi and MacEwen classification system was used for evaluation of the AVN. Avascular necrosis was found in 35,9% of the treated hips. We established the influence of several radiological and clinical data on the incidence and severity of AVN. Conclusion: In our analysis the degree of initial dislocation according to Tönnis classification is an important risk factor for AVN. Age at the onset of treatment, presence and size of ossific nucleus, the use and period of preliminary traction, previous treatment with Frejka pillow or Pavlik splint, sex and side were not associated with the incidence and severity of ischemic necrosis. The results have been analysed statistically

To review the results of limb lengthening and deformity correction in fibular hemimelia, fifty-five patients with fibular hemimelia underwent limb reconstruction at Sheffield Children’s Hospital. According to Achter-man and Kalamchi classification, twenty-six were classified as Type IA, six as Type IB and twenty-three as Type II fibular hemimelia. All patients had at least some shortening of ipsilateral femur but forty-nine had sig-nificant femoral deficiency. Lengthening of tibia and in significant cases femur was done using De Bastiani or Vilarrubias or Ilizarov methods. Ankle valgus and heel valgus were corrected through osteotomies either in the supramalleolar region or heel. Equinus was corrected by lengthening of tendoachelis with posterior soft tissue release and in severe cases using Ilizarov technique. The average length gained was 4.2 cm (range 1 to 8) and the mean percentage of length increase was 15.82 (range 4.2 to 32.4). Mean bone healing index was 54.23 days/cm. Significant complications included knee subluxation, transient common paroneal nerve palsy, and recurrence of equinus and valgus deformity of foot. Overall alignment and ambulation improved in all patients. Knee stiffness due to cruciate deficient subluxations needed prolonged rehabilitation. Presence of 3-ray foot gives a better functional result and cosmetic acceptance by patients. The Ilizarov frame has the advantage to cross joints and lengthen at the metaphysis. Limb reconstruction in fibular hemimelia using limb lengthening and deformity correction techniques improve functional status of involved lower limb

The results of the Ferguson medial open reduction of the hip for DDH were reviewed to determine the complications, re-operation rate, clinical and radiological outcome. Notes were reviewed for 75 cases, of which 5 were bilateral. X-rays were available for 69 hips and were analysed for Acetabular index (AI) and Centre Edge (CE) angles of the operated and unaffected hips. The hips were assessed for avascular necrosis by the method of Kalamchi and MacEwan and were graded according to Severin. The mean age at operation was 11.8 months (range 3-23, SD 4.42, mode 11). The mean clinical follow up was 65.1 months (range 4-148, SD 33.4). The mean radiological follow up was 58.2 months (range 3 – 131, SD 31). No further surgical procedure was required for 60 hips (75%). Of the remainder, a Salter osteotomy was performed for 8 hips, of which 6 had additional procedures. 8 hips required a femoral osteotomy, 2 an Arthrogram and one a triple pelvic osteotomy. The AI improved following surgery, with a rate of increase double that for the unaffected side. The mean centre edge of the operated side was 6 degrees less than the unaffected side. Tables 2 and 3 show that the majority of hips had no avascular necrosis and a good radiological outcome. The results compare favourably with the literature. The conclusion is that the Ferguson medial open reduction has good long-term results with low rates of avascular necrosis