The morphologic features of hips with congenital dysplasia differ from those of normal hips. Dysplastic hips have a shallow, vertically inclined acetabulum and a subluxated or even dislocated femur with a straight, narrow canal and a short, excessively anteverted neck.6-8,12,28,31,36,37 Findings using computed tomography (CT)29 suggest the primary deformity of the dysplastic femur is rotational, with a highly variable neck-shaft angle depending on the degree of subluxation. The rotational deformity of the dysplastic femur arises in the diaphysis between the lesser trochanter and the isthmus, and is not attributed to a torsional deformity of the metaphysis.29 Given the general belief that fit and fill are important,3,11,14,21 custom-designed femoral prostheses,1,2,24,26,27,32,35,38,39 modular prostheses,17 and hybrid total hip arthroplasties (THA)22 have been advocated for treating severe osteoarthritis (OA) secondary to congenital hip dysplasia (CHD). To achieve the optimal fit and fill in patients with variable femoral shapes, some authors think custom-designed femoral prostheses or modular prostheses are more likely to be effective than off-the-shelf components.1,26,27,35,38,39
The short-term results of cementless custom-designed femoral components have been reported to be no better, and sometimes worse, than off-the-shelf stems.2,24,32 The reasons for worse results might relate to other aspects of design such as surface finish and amount or location of area for bone ingrowth.2,24,32 Smooth surfaces with longitudinal grooves lead to deterioration of the bone-implant interface and clinical failure.24,32 Proximal titanium mesh pads that are not circumferential may be insufficient to secure fixation.2 However, whether alternative surface finishes will influence these results is unknown.
The effect of surface finish on stability has been recognized and incorporated in various ways.3,5,9,14,15 A corundum-blasted surface had good efficacy15 and allowed bone ongrowth for biologic fixation in a 5-to 10-year multicenter study.9
Therefore, we asked the following questions: (1) whether cementless custom-designed femoral components with sandblasted surfaces would provide high survival rates, (2) what percentage of the custom femoral components would acquire bone ongrown fixation, and (3) which factors, including Crowe type,7 canal fill, and complications, would influence the radiographic mode of stem fixation.
MATERIALS AND METHODS
We prospectively evaluated 78 consecutive patients with 100 custom-designed total hip femoral components implanted for degenerative OA secondary to CHD from January 1994 to July 1997. Patients with OA as a result of CHD were treated with custom-designed femoral components implanted by two surgeons (KO and NS). One patient (one hip) was lost to followup. Therefore, 77 patients (99 hips or 99%) were available for clinical and radiographic evaluations. We thought that the results of 99 hips were sufficient to be analyzed statistically and be compared with published results for cementless custom-designed femoral components. There were 70 women and seven men. The mean age of the patients at the time of surgery was 54 years (range, 40-73 years), and their mean height was 152.2 cm (range, 130-175 cm) and mean weight was 54.8 kg (range, 38-73 kg). The mean body mass index (BMI) was 23.6 (range,17.3-30.6). According to the classification system of Crowe et al,7 47 hips had Type I deformity (< 50% subluxation), 41 hips had Type II deformity (50-75% subluxation), and 11 hips had Type III deformity (75-100% subluxation). Patients with Crowe Type IV deformity were excluded and all were treated with a modular system. Nine hips (nine patients) had previous femoral osteotomies, four hips (four patients) had varus osteotomies, and five hips (five patients) had valgus osteotomies. Four hips (three patients) had previous acetabular osteotomies, three hips (two patients) had Chiari osteotomies, and one hip (one patient) had a shelf operation. The minimum followup was 7 years (mean followup, 9 years 3 months; range, 7-11 years).
Custom-designed femoral components (Cremascoli, Milano, Italy) (Fig 1) were produced using a CAD/CAM system based on CT data. The inner and outer contours of the femur were digitized on serial 3 to 5-mm pitch axial CT images, after which an implantable femoral component with maximum proximal canal filling was designed. The mean stem length was 121 mm (range, 103-135 mm). After femoral component insertion was simulated on a computer, a curved anatomic shape was made from titanium alloy (Ti-6Al-4V). Semicylinder-shaped surface grooves with a radius of 0.5 mm were added to the implant in a 5-mm grid to enhance mechanical locking onto the bone, femoral components were sandblasted with mesh sand (Al2O3, 106-250 μm) under 4 bar pressure for a few minutes at room temperature to provide a4.95 ± 1-μm surface finish.
The operations were performed with the patients under general anesthesia and using a posterolateral approach without trochanteric osteotomy. A total capsulectomy was performed to increase ROM and allow an increase in limb length. The femoral canal was opened with a custom-designed broach that precisely duplicated the shape of the custom-designed prosthesis to provide an exact fit. After inserting the femoral components, a modular neck and head system34 was used to adjust limb-length discrepancy, femoral offset, and femoral anteversion (Fig 2). Femoral offset was altered in 90 hips (91%): neutral short/long necks were used in 84 hips, varus/valgus short/long necks were used in three hips, and lateralized/medialized short/long necks were used in three hips. Femoral anteversion was planned from 15° to 30° and was altered in nine hips (9%): 15° anteverted/retroverted short/long necks were used in five hips, and monoblock retroverted necks with custom implants were designed for four hips with femoral anteversion greater than 45°. The modular head system consisted of three increments of alumina-ceramic heads (Biolox®, CeramTec, Plochingen, Germany); short heads (-3.5 mm) were used in 27 hips, medium heads (0 mm) were used in 48 hips, and large heads (+3.5 mm) were used in 24 hips. The diameter of the modular head was 28 mm in all hips.
On the acetabular side, a metal cancellous cementless socket and a polyethylene cup insert (ESKA, Lübeck, Germany)25 were used in 83 hips. The metal cancellous cementless socket was manufactured from cobalt-chrome molybdenum alloy. The entire surface in contact with bone has a structure like cancellous bone.25 This metal cancellous surface was produced as an integral part of the implant with an intercommunicating porous structure with a pore size ranging from 800 to 500 μm and 60% porosity. Initial stability was achieved by two spikes wedged into the anterior and posterior parts of the acetabular rim, and a peg embedded into the ischium. Of these 83 hips, we used a 10°-elevated polyethylene liner in 25 and a flat liner in 58. In the remaining 16 hips (11 patients) having surgery from September 1996 to July 1997, we used an ANCA-fit socket and alumina-ceramic insert (Cremascoli) that became available after September 1996. The ANCA-fit socket was made of Ti-6Al-4V, and the entire surface in contact with bone had a porous coating. Initial stability was achieved by two spikes wedged into the anterior and posterior parts of the acetabular rim. The average outer diameter of the acetabular component was 48 mm (range, 44-56 mm). Although bulk structural bone grafts were not used, morselized autogenous bone was retrieved from the acetabular reamings and used as nonstructural grafts to supplement the uncovered superolateral part of the ilium above the acetabular component. Morselized autogenous bone grafts also were used to fill cystic acetabular lesions.
All patients received prophylactic intravenous antibiotics (2 g second-generation cephalosporins at the time of anesthesia, then 2 g every 12 hours for 72 hours). Anticoagulants were not administered. Elastic stockings were used to prevent thromboembolic events for the first 2 postoperative weeks. Patients without complications were allowed partial weightbearing at 1 week and full weightbearing at 3 weeks postoperatively. Patients with any intraoperatively incurred cracks or fractures were instructed in partial weightbearing using crutches for 6 weeks.
Clinical and radiographic data were evaluated by two independent authors (SL and TN) blinded to the results. Clinical results were evaluated preoperatively and at the latest followup using the Harris hip score (HHS).20 The presence of thigh pain also was evaluated at the latest followup. Anteroposterior (AP) and true lateral radiographs were taken preoperatively and at each followup. The mode of femoral component fixation was evaluated radiographically 2 years postoperatively according to modified criteria of Engh et al,14 bone ongrowth fixation, stable fibrous fixation, and unstable fixation. Femoral component migration was determined using the vertical distance from the shoulder of the stem to the midpoint of the lesser trochanter and the varus angle of the stem formed by the stem axis and the proximal femur axis. Greater than 4 mm subsidence vertically5 or changes greater than 2° in the varus angle indicated stem migration and loosening. Acetabular component loosening was defined as greater than 2 mm migration or greater than a 5°-change in the abduction angle of the acetabular component.5 Cortical index and canal flare index28 also were evaluated. Canal filling seen on the AP and lateral radiographs 3 weeks postoperatively was measured 1 cm below the lesser trochanter level and 1 cm above the stem tip level. We also measured the presence of radiolucent lines in each femoral Gruen zone18 and acetabular zone,10 cortical hypertrophy,14 proximal stress shielding,13 ectopic bone formation,4 and femoral osteolysis. Femoral osteolysis was defined as areas of endosteal, intracortical, or cancellous loss of bone that was scalloped or had the appearance of destruction of bone rather than disuse osteopenia.22
We measured the abduction angle to assess acetabular component position. The hip center was defined as the center of the prosthetic femoral head. We then measured the distance between the interteardrop line and the hip center. The magnification ratio for each radiograph was determined by measuring the diameter of the prosthetic femoral head and dividing it by the known diameter of the femoral head (28 mm). Linear head penetration into the polyethylene liner was not measured because the entire outline of the ceramic head was not visible in the metal acetabular cup on the plain radiographs. We also measured the preoperative and postoperative limb-length discrepancies.
Kaplan-Meier survival analysis was used to calculate the probability of retention of the original prosthesis with 95% confidence intervals (CI) (StatView; SAS Institute, Cary, NC). Statistical analysis for comparison between the two groups using the modified criteria of Engh et al14 for various factors was performed. Differences in age, height, weight, BMI, HHS, cortical index, canal flare index, stem length, and canal fill between the two groups were analyzed statistically using the Mann-Whitney U test. Differences in gender, the number of hips in patients with thigh pain, and complications were analyzed statistically using Fisher's exact probability test. Differences in Crowe's classification were analyzed statistically using the chi square test. A p value less than 0.05 was considered significant.
Kaplan-Meier survival analysis with revision surgery as the end point showed a 99% probability (95% CI, 0.97-1) of retaining the femoral component 9.3 years postoperatively and a 100% probability of retaining the acetabular component. One femoral component was revised because of aseptic loosening at 6 years postoperatively. No acetabular components were revised. The mean total HHS improved from 42 points preoperatively to 98 points at the latest followup (Table 1). At the latest followup, seven patients (seven hips) had thigh pain; three patients (three hips) consistently had pain on walking, and four patients (four hips) had occasional slight pain on walking only and no pain at rest. Three (3%) intraoperative femoral fractures were wired. There was one (1%) dislocation. No patients had sciatic nerve palsy or deep venous thrombosis.
Eighty-eight hips (89%) had femoral component bone ongrowth fixation 2 years postoperatively (Table 1; Fig 3). These had bone ongrowth onto the middle part of the stem. Seven hips (7%) had stable fibrous fixation. Four hips (4%) had unstable fixation and loosening, one with varus migration and three with greater than 4 mm subsidence (Table 2). The mean proximal canal filling seen on AP radiographs was 91.8%, and the mean filling seen on lateral radiographs was 87%. The distal corresponding values were 77.4% and 73.9%, respectively. A radiolucent line around Zones 1, 3, 4, and 5 of the femoral component was seen in 16 to 27% of hips. There was a radiolucent line around the acetabular component in 10 hips (10%) for Zone 3. Proximal stress shielding occurred in 56 hips (57%); cortical hypertrophy occurred in three unstable hips (3%); and ectopic bone formation was seen in 10 hips (10%). Osteolysis was identified around one femoral component (1%) and one acetabular component (1%). No patients were awaiting revision surgery for wear and/or osteolysis. The mean abduction angle of the socket was 46.7° (range, 35°-60°). In 81 hips (82%), the cup abduction angle was within 50°. The hip center was an average of 25 mm (range, 14-41 mm) proximal to the interteardrop line. Bulky structural bone grafts were not used. The mean preoperative limb-length discrepancy was 14 mm (range, 0-5.8 cm), whereas the mean postoperative limb-length discrepancy was 4 mm (range, 0-6.3 cm). Seven hips had a limb-length discrepancy greater than 1 cm because the contralateral side was the untreated side of hips with Crowe Type II or Type III deformity.
A comparison of the bone ongrowth hip group with the stable fibrous hip and unstable hip group revealed a decreased (p = 0.009) postoperative HHS, an increased (p = 0.009) number of patients with thigh pain (p =0.0001), an increased (p = 0.032) number of hips with intraoperative fractures, and an increased (p = 0.0001) number of hips with varus malposition of the stem in the stable fibrous hip and the unstable hip group (Table 3). Crowe's classification, cortical index, canal flare index, or canal fill, BMI, preoperative HHS, did not influence whether the hips had stable or unstable fixation.
Anatomic cementless custom-designed femoral components with blasted surfaces provided better clinical and radiographic results in patients with OA secondary to CHD than cementless custom femoral components with a surface finish unsuitable for bone ongrowth.2,24,32 Custom femoral components with blasted surfaces had better survival rates at longer-term followups than the short-term results of the other custom femoral components.2,24,32
Our study has several limitations. First, we did not have any directly comparable control group. However, our results are comparable with published clinical and radio-graphic results of other custom-made femoral components. Our patients weighed less and were shorter than a comparable group of American or European patients with OA of the hip,32 possibly influencing the high survival rate. Second, the patients had OA secondary to CHD. Nine hips had previous femoral osteotomies; however, these previous osteotomies did not seem to influence the survival rate because these hips showed bone ongrowth fixation. Third, interobserver and intraobserver variations in clinical and radiographic evaluations could not be checked. Finally, although we performed custom THAs in 77 consecutive patients with secondary OA with CHD, patients with Crowe Type IV deformity were excluded in our prospective study and were treated with a modular system.
Treating patients with OA of the hip using cementless custom-designed femoral components has produced variable results depending on the implant design and surface finish (Table 4). Since an intraoperative custom-molded femoral component was first introduced,27 some clinical studies24,32 have failed to show any substantial improvement in clinical success or implant longevity for custom-designed prostheses. This may relate to their smooth surface finish with longitudinal grooves. Titanium custom-designed stems with proximal titanium mesh pads1,35 have not produced better results than off-the-shelf stems. The pads sometimes separated from the stems probably because the circumferential proximal titanium mesh pad was insufficient to acquire secure fixation. In contrast, 62 hips (in 57 patients with OA of the hip) with custom-designed femoral stems with a porous hydroxyapatite coating in the proximal 2⅔ reportedly showed no migration or subsidence at a mean followup of 94.9 months.38 The effect of the prostheses' surface finish on stability has been recognized and incorporated in various ways, such as plasma spray coating on titanium alloy,3 porous-coated stems,5,14 and grit-blasted titanium alloy.9,15 In a 5-to 10-year multi-center study using grit-blasted titanium tapered stema, the survivorship at 10 years with definitive aseptic loosening as the end point was 100% in 118 THAs.9 In our study, custom-designed femoral components with a sandblasted surface had a 99% survival rate at a mean followup of 9 years 3 months in 99 hips with OA secondary to CHD.
Four hips (4%) were unstable. Intraoperative fracture and varus malposition of the stem occurred more frequently in the stable fibrous hip and the unstable hip group. Even if the sandblasted surface finish was appropriate for bone ongrowth, the intraoperative technical errors, including intraoperative fracture and varus malposition of the stem, compromise or degrade fixation to bone. Patients with CHD have a severe lateral curvature of the proximal canal. A 120-130-mm long stem with maximal filling for the femoral canal may be too long for insertion in hips with CHD.33 These relatively long stems have not allowed a large distal dimension to be implantable. These factors may be related to the technical errors.
We think this modular neck and modular head system was effective in these custom-designed femoral components for patients with CHD for three reasons. First, this system is able to adjust limb-length discrepancy and muscle tension, even if an acetabular component is fixed to the high position of the pelvis as in hips with Crowe Type II or Type III deformity. Second, the system is applicable to various amounts of femoral anteversion and femoral offset seen in patients with CHD. Third, the system maximizes range of motion and minimizes impingement. Both may contribute to decreasing the prevalence of dislocation. Even if dislocation does not occur, the hard-on-hard bearing couples in THA make it increasingly important to prevent excessive impingement between the metallic stem and the acetabular insert rim to prevent chipping of the ceramic insert or metal neck breakage.19 The 1% dislocation rate was less than rates reported in previous series of patients with CHD,16,22,23,30 despite using the posterior approach without repairing the external rotators and the posterior aspect of the capsule.
To eliminate varus malposition of the stem resulting from insufficient insertion and intraoperative femoral fracture, and to acquire more distal canal fill, we changed the femoral component design and reduced the stem length.33 Second-generation custom-designed femoral components with 100-mm stem length have been used for treatment of patients with OA of the hip secondary to CHD since 1998. Anatomic cementless custom-designed femoral components with blasted surfaces provided favorable results in patients with Crowe Type I, II, or III deformities if technical errors (eg, intraoperative fractures and stem malposition) do not occur.
We thank Hidenobu Miki, MD, PhD, for technical support.
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