We retrospectively identified 77 patients from four centers who had pelvic discontinuity treated with a custom Pinnacle™ Triflange Acetabular System (DePuy Orthopaedics, Inc, Warsaw, IN, USA) (Fig. ) from 1992 to 2008. All data had been prospectively collected. During the study period, all patients with osteolytic pelvic discontinuity were treated with triflange acetabular component at the authors’ institutions. The indication for the triflange acetabular components was osteolytic pelvic discontinuity in the setting of failed THA. The contraindications were (1) active infection, (2) medical comorbidities inhibiting operative intervention, and (3) bone loss that was so severe that fixation for the triflange component could not be achieved. Twenty of these 77 patients were reported in a previous article [5
] at 89 to 157 months’ followup. Eleven patients were lost to followup. This left 66 patients for review. Nine of these 66 patients died without radiographs performed at greater than 2 years’ followup, but all had a least 2 years’ clinical followup. This left 57 patients for review with complete minimum 2-year clinical and radiographic followup. The average age of the 57 patients was 61 years (range, 35–81 years). There were 51 women and six men. The average BMI was 27 (range, 21–40). All patients had had a minimum of one previous hip arthroplasty procedure. The minimum followup was 24 months (average, 76 months; range, 24–215 months). IRB approval was obtained by the authors for all institutions involved.
Fig. 1A–C (A) A preoperative AP pelvic radiograph demonstrates a failed acetabular component with pelvic discontinuity. (B) A 3D reconstruction of CT scans demonstrates the anatomy of the discontinuity. (C) A postoperative AP pelvic radiograph demonstrates a well-fixed (more ...)
Preoperative bone deficiency was classified by the operating surgeon according to the method of the American Academy of Orthopaedic Surgeons (AAOS) as reported by D’Antonio et al. [4
] using AP radiographs of the pelvis and AP and true-lateral radiographs of the hip. Pelvic discontinuity (AAOS Type IV) is defined as a defect across the anterior and posterior columns with total separation of the superior from the inferior acetabulum. Trochanteric escape was identified by the authors as nonunion of any nature of the greater trochanter to the femoral prosthesis or remaining femur with greater than 1 cm of displacement.
The first goal of acetabular reconstruction with the triflange cup was to achieve initial stable fixation through intimate contact between structural host bone and the rigid iliac, ischial, and public flanges augmented with multiple 6.5-mm screws. The second goal was to achieve fixation through this intimate contact of the host bone and the flanges and posterior hemispherical cup augmented with porous surfaces. The design of the triflange cup was initiated with a CT scan of the patient’s pelvis. The standard CT scan of the pelvis consisted of 3-mm cuts of the pelvis, with metal subtraction software with the uncompressed data recorded on a CD-ROM and sent to the implant manufacturer. The CT scan slide data were translated to create a computerized, three-dimensional (3D) reconstruction of the patient’s hemipelvis and a one-to-one hemipelvis model was constructed.
The remaining pelvic landmarks (obturator foramen, iliac wing, pubic ramus) were then used to determine the hip center, cup orientation, and flange geometry and to identify thin, fragile bone along the remaining rim of acetabulum that was removed at the time of cup insertion. This incompetent bone was removed from the 3D model before the triflange cup was designed. Using the markings of the flanges made on the pelvic model, a clay prototype of the cup was prepared. This assisted the surgeon in determining the head center and cup orientation. The head center location was chosen based on patient-specific considerations, including leg length discrepancy, planned retention or revision of the femoral component, length of contralateral leg, and cup size. Generally, the vertical head center location was established by first determining the approximate anatomic position of the head center using the superior aspect of the obturator foramen as a reference point. The remaining bone of the anterior and posterior columns determined the head center in the coronal plane, whereas the flange geometry and cup face diameter guided the position of the head center in the sagittal plane. The cup face orientation was established by setting the abduction and anteversion angles of the cup. The abduction angle generally was targeted at 35° to 40° from horizontal and was established using the plane of the obturator foramen as a reference. The anteversion angle was established using the plane of the iliac wing and the obturator foramen as references. The three flanges were then designed to facilitate initial fixation. The first row of screw holes targeted the most inferior, structural bone of the ilium. Care was then taken to achieve fit on the two planes of the ilium delineated by the gluteal ridge. The ischial flange normally had three to seven screw holes to accommodate 6.5-mm acetabular screws and was designed to rest primarily on the posterior surface of the ischial tuberosity. The pubic flange generally was the smallest of the three and normally did not contain screw holes. Once the design of the implant was finalized, reverse-engineering techniques were used to digitize the surface of the clay prototype into a numerical format used by computer-controlled machining centers to mill the surfaces of the titanium stock. The blank of wrought titanium bar stock was prepared using a hemispheric inner geometry compatible with standard, snap-in UHMWPE acetabular liners. This blank was fixed in a five-axis mill to machine the surfaces of the device. Porous coatings were applied to the medial aspect of the flanges and the cup portion of the device to facilitate osteointegration. Clearance was built into the medial aspect of the junctions between the flanges and cup portion to compensate for any discrepancies between the CT-generated hemipelvis model and actual patient anatomy. This ensured the device transferred loads through the broad faces of the flanges to the cortical bone of the ilium, ischium, and pubis, rather than to the deficient bone of the acetabular rim.
The hip generally was approached with a standard posterior incision. An extended trochanteric osteotomy was used in selected cases, primarily for difficult femoral stem or cement removal. The sciatic nerve was identified and traced from the greater sciatic notch to the ischium. After dislocation of the femoral head, the femoral stem was removed. In cases where the femoral component was retained, the gluteus minimum and gluteus medius were elevated and a space created between the muscle and the ilium. The femoral head was then displaced into this space. The gluteus medius and minimus were elevated off the wing of the ilium, taking care to protect the superior gluteal artery and nerve as they exited the greater sciatic notch. The hamstring tendon origin was released sharply from the ischium as far as needed to facilitate the ischial flange placement. Particular care was taken to protect the sciatic nerve during this maneuver because it lies in close proximity. Finally, a pouch was created over the pubis to accept the pubic flange. The gas-sterilized, 3D pelvic model was referenced intraoperatively, and the thin, fragile rim of bone in the periacetabular area was removed to match the bone removed on the hemipelvis model as determined preoperatively. Bony defects were supplemented with particulate allograft. Insertion of the triflange cup usually was initiated with insertion of the ilial flange, which was facilitated by translating the hip proximally with some flexion to relax the abductors. The ischial and pubic flanges were rotated into position while extending the hip. If remaining bone was found that needed to be contoured further, the triflange was removed and further burring was completed per the pelvic model. Fixation was initiated with screws in the ischial flange, where the bone is poorest and lysis is common. The ilial flange then was fixed with screws, again protecting the superior gluteal vessels, and the liner was inserted. It is important to note the deep branch of the superior gluteal artery and superior gluteal nerve traverses 4 to 6 cm superior to the acetabular rim [12
]. In cases of discontinuity, the iliac screws pulled the flange down into intimate contact with the bone, which reduced the discontinuity and rotated the inferior ½ of the hemipelvis into correct orientation relative to the superior hemipelvis. Nine to 13 screws generally were inserted. The average acetabular cup size was 55 mm (range, 48–68 mm). Twelve hips had constrained liners placed at the time of index operation. Three hips had femoral heads of 40 mm or greater placed at the index operation (Fig. ).
Postoperatively, patients were mobilized with the use of a walker in the acute hospitalization, usually on the first postoperative day. Physiotherapy consisted of ambulation with the use of a walker with partial weightbearing. Muscle strengthening exercises for the hip were held until 6 weeks postoperatively. Weightbearing was restricted for 12 weeks, allowing toe touch weightbearing with a walker or crutches. After 12 weeks, the patients were allowed to progress to weightbear as tolerated over a four 4 week period. The patients were instructed in gently daily hip strengthening after 12 weeks. Patients received supervised therapy based on the operative surgeon’s discretion.
The followup of patients varied among centers, but all patients were evaluated within 12 weeks after surgery and at 1 year and then followup varied based on surgeon discretion. At each visit, clinical results were obtained by the treating physicians using the HHS [7
]. All patients received an AP pelvis radiograph. Strength grading was as follows: 0 = absent voluntary contraction, 1 = feeble contractions that are unable to move a joint, 2 = movement with gravity eliminated, 3 = movement against gravity, 4 = movement against partial resistance, and 5 = full strength [6
One of the five of us (MJT, TKF, MJC, GEH, TB) reviewed preoperative and postoperative radiographs at each of our respective institutions. The average radiographic followup was 76 months for all patients included in the study. Early and most recent postoperative films were reviewed and compared for the presence of radiolucent lines; evidence of bony remodeling and healing of pelvic discontinuity; and evidence of loosening, migration, screw breakage, or screw motion. Utilizing the criteria of Berry et al. [2
], we considered the pelvic discontinuity healed if trabecular bone or callus bridged the discontinuity and we considered the discontinuity unhealed if the fracture line still was visible or if there was evidence of loosened or broken screws. Migration was assessed by comparing immediate postoperative and most recent radiographs and looking for any movement of the triflange or screws. Any movement of the implant or screws of more than 2 mm was deemed migration. Screws were scrutinized for any “pullout” or “halos” indicative of loosening. We considered components to have definite loosening if there was acetabular migration of 2 mm or more in the horizontal or vertical direction with implant rotation, screw breakage, or progressive bead shedding. There was probable loosening if there was a radiolucent line of more than 1 mm in all three zones without migration, rotation, or screw breakage [11
]. We defined stable fixation as documented stable fixation of the triflange component with time to the ilium and superior acetabulum with or without healing of the discontinuity and migration of the inferior pelvis.
We compared the cost of a custom triflange implant with other commonly reported methods of treating a pelvic discontinuity, specifically a Trabecular Metal® cup combined with a antiprotrusio cage, the “cup-cage” construct (Zimmer, Inc, Warsaw, IN, USA), and the “distraction technique” that commonly utilizes a Trabecular Metal® cup, combined often with two Trabecular Metal® wedges for supplemental fixation (Zimmer). We performed this cost comparison by asking each manufacturer the cost of the implants.
Descriptive statistics including frequency, proportion, mean, and range were calculated.