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The S-ROM Modular Femoral Stem in Dysplasia of the Hip
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Abstract
Abnormal femoral neck anteversion, previous proximal femoral osteotomies, and small femoral canals with metaphyseal/diaphyseal mismatch are commonly seen in patients with developmental dysplasia of the hip (DDH) and are ideal indications for using a modular femoral stem. The torsional stability of the fluted modular stem makes it the implant of choice for subtrochanteric osteotomy stabilization for reduction of high-riding DDH. Modularity optimizes proximal and distal implant stability while permitting adjustments to anteversion, offset, and leg length to provide custom biomechanical reconstruction of the DDH hip. Clinical results with few complications can be achieved when using this stem in complex DDH surgery.
Elements of developmental dysplasia of the hip (DDH) occur frequently in primary total hip arthroplasty (THA).1-5 A review of 75 hips with idiopathic osteoarthritis revealed proximal femoral deformity in 40% of hips and acetabular dysplasia in 39% of hips.4 The variable shape of the femoral canal (canal/flare index) may cause difficulty in achieving a proximal and distal fit with standard femoral components.6 Although modular femoral stems are useful in most hips undergoing primary THA, they are especially effective in hips with proximal femoral deformities. Abnormal femoral neck anteversion, a previous proximal femoral osteotomy, and small femoral canals are indications for using a modular femoral stem. Small-diameter stems are frequently required for DDH hips. Press-fit modular stems avoid the complications of thin cement mantels and subsequent cement fractures. Press-fit modular stems also reduce the chance of cementless stem fracture development because the stem is not porous coated.7 The torsional stability of modular stems is equivalent to the torsional stability of cemented stems, which is another reason that modular stems are effective in subtrochanteric osteotomy stabilization for reduction of high-riding DDH.8 Contraindications to the modular stem are rare but may include an extreme femoral canal deformity, a condition in which cemented or custom stem fixation is more easily achieved.
Preoperative Planning
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A detailed patient history, physical examination, and radiographic evaluation are required for patients undergoing primary THA. The history of patients with DDH should focus on prior treatments, surgeries, and complications. The patients’ conditions (eg, leg length, fatigue, limp) and pain patterns should be thoroughly discussed.
Physical examination often reveals abnormalities in size, range of motion (stiffness or laxity), leg lengths, and prior incisions, which can cause difficulty in surgery. Surgeries of the contralateral limb, such as epiphysiodesis, should be noted. Leg lengths are assessed by using a tape measure and blocks under the short limb to determine exact discrepancies including the length that provides the best balance for the pelvis. Thorough preoperative assessment of femoral and sciatic nerve function is essential.
Plain radiographs should include anteroposterior (AP) radiographs of the pelvis and AP and Lauenstein lateral radio-graphs of the involved hip. Radiographic magnification markers taped to the involved hip yield an accurate estimate of radiograph magnification, permitting precise femoral canal sizing and templating. Computed tomography scans are rarely indicated but can provide more accurate assessment of anteversion, femoral canal dimensions, and acetabular bone stock assessment than plain radiographs. Scanograms may be useful in assessing limb length inequalities more accurately than radiographs.
Surgery
During surgery, a patient is positioned laterally with the DDH hip raised. The underlying leg is placed in flexion to reduce the degree of lumbar lordosis. The trunk and pelvis are appropriately stabilized while the operated leg is prepped and draped free over a radiolucent table to permit fluoroscopic evaluation.
Anterolateral, direct lateral, posterior, and transtrochanteric approaches to the DDH hip can be used. The author prefers the posterior approach to avoid scars from prior anterior surgeries (which are common), to minimize damage to the abductors, and to facilitate identifying, protecting, and monitoring the sciatic nerve. The posterior approach can be converted to a trochanteric or subtrochanteric osteotomy for patients with stiff or high-riding DDH.
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In thin, flexible hips requiring <1 cm limb lengthening, a small incision is made from the midpoint of the vastus tubercle and extends proximally and posteriorly for 4-6 inches. The incision is extended further proximally and distally in patients who are relatively large and/or stiff, especially in patients requiring trochanteric or subtrochanteric osteotomies or limb lengthening >1 cm. The fascia lata and gluteus maximus are divided in line with the incision. The sciatic nerve is identified by palpation but not dissected. Partial or full release of the gluteus maximus tendon at the linea aspera is performed to prevent tethering of the sciatic nerve during leg manipulations and lengthening. The external rotators are released separately from the posterior capsule and reflected posteriorly to protect the sciatic nerve, which often lies just lateral to the ischium. Although posterior capsulotomy and repair is preferred for Crowe I DDH hips (Figure 1), capsulectomy is generally preferred for Crowe II, Crowe III, and Crowe IV hips require further exposure and limb lengthening. A smooth 7-16 inch Steinman pin is placed in the ischium at the level of the transverse ligament for limb length assessment prior to dislocation.
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The hip is dislocated posteriorly if it is not already severely subluxed or dislocated (Crowe III, Crowe IV).2 The greater and lesser trochanters and femoral head are used as landmarks in conjunction with preoperative templates to determine the level of femoral neck osteotomy. When a subtrochanteric osteotomy is performed to help reduce the hip (Crowe III, Crowe IV), the femoral canal is prepared distally and proximally, and the trial sleeve is positioned in the proximal femur (Figure 2).9 The vastus lateralis is then reflected from the vastus tubercle distally for 6-10 cm. The linea aspera is identified, and rotation marks are made on the femur before the osteotomy. The iliopsoas tendon is sectioned just proximal to its insertion at the lesser trochanter. A transverse osteotomy is then made distal to the sleeve and approximately 3.5 cm distal to the lesser trochanter (Figure 3). The acetabulum is then easily exposed by complete capsulectomy and anterior displacement of the proximal femoral fragment and attached abductors. If a subtrochanteric osteotomy is not performed (Crowe I and Crowe II), then the capsule is dissected inferiorly until the transverse ligament and true socket are identified. The acetabulum is then prepared and positioned. Acetabular bone deficiencies may require cup placement to be more medial, higher, or in a more abnormal version than desired. The advantage of the modular stem is that it can acclimate to abnormal socket positions by increasing offset, neck length, or independent version of the stem from the sleeve to maximize myofascial tension, leg length, and stability.
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Figure 4: Cylindrical diaphyseal reamers in 0.5-cm increments prepare the diaphysis until firm cortical contact is achieved. Figure 5: Conical reamers in 2-mm increments prepare the metaphyseal bone until firm anteroposterior contact is achieved. The distal pilot corresponds in size to the last cylindrical diaphyseal reamer used. Figure 6: The triangle calcar miller prepares the metaphyseal flare in the version that allows maximum contact with host bone. The distal pilot and proximal cone pilot correspond to the final distal and proximal reamers previously used. |
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The femoral canal is identified with a box osteotome and canal finder. A three-step milling process then prepares the femoral canal. Step one involves cylindrical diaphyseal reaming until firm endosteal cortical contact is achieved to prepare the distal femur (Figure 4). Because DDH femoral canals are often small, surgeons should begin with the smallest diameter reamer and increase in 0.5 to 1.0-mm increments. The final reamer should be the same width as the minor diameter of the chosen stem or be 0.5 mm larger than the minor diameter of the chosen stem. If a subtrochanteric osteotomy has been performed, then reamers can be placed into the distal bone fragment through the osteotomy site of a depth that matches or exceeds the final stem placement after excision of the subtrochanteric fragment.
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The proximal femur is prepared in two steps. A shaft pilot, matching the minor stem diameter, directs proper placement of the proximal reamers. Conical metaphyseal reamers in 2-mm increments are placed until firm anteroposterior proximal diaphyseal and metaphyseal contact are obtained without excessive thinning of the proximal endosteal cortex (Figure 5). Calcar miller reamers are then used to mill the metaphyseal flare to maximize host bone contact irrespective of sleeve version (Figure 6).
Femoral sleeve and stem trials are then placed. Ten sleeve options are available for each diameter stem. The trial neck can be rotated in 10° increments until the desired stem anteversion is achieved (Figure 7). Femoral neck and head options are then selected to provide desired leg length and offset.
Trial reduction is performed to assess leg length, stability, combined anteversion, and range of motion. If limb lengthening has been performed, then the initial reduction should begin with the shortest length and offset trial slowly increased as desired. Soft tissue releases of the capsule, gluteus maximus tendon, iliotibial band, tensor fascia lata, and straight head of the rectus femoris tendon and iliopsoas tendon are performed until desired tissue tension and leg length are achieved. When a subtrochanteric osteotomy has been performed, the trial stem and sleeve are placed into the proximal fragment and reduced into the acetabular component with the leg in full extension. An assistant then distracts the distal fragment and the amount of overlap between the distal and proximal bone fragments determines the amount of initial subtrochanteric bone to be resected. A second transverse osteotomy is made in the distal fragment. Trial reduction corrects anteversion abnormalities through stem placement or derotation of the osteotomy fragment. A Lohman bone clamp stabilizes the fragments during trial reduction and implant insertion.
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Differences in stem and sleeve anteversion are noted upon trial implant removal. The real sleeve is then inserted into the prepared proximal femur, and the stem is introduced in the proper amount of anteversion (Figure 8). Final reduction is performed with a desired combined anteversion of 40°-45°. If a subtrochanteric osteotomy is performed, then rotational stability is assessed. A unicortical plate or cortical-only allograft (Figure 9) is applied if the osteotomy is not stable rotationally.
Postoperative Management
Patients begin ambulation with a walker on the first or second postoperative day and progress to 50% weight bearing with crutches for 4 weeks followed by weight bearing as tolerated with two crutches for 2 weeks. All patients are re-evaluated at 6 weeks postoperatively. If progress is appropriate, then strict hip precautions are eliminated and progressive range of motion, biking, and strengthening are permitted. Weight bearing as tolerated with a cane for 3 more weeks is encouraged. All patients are informed that they will improve in flexibility, strength, endurance, and gait until 1-2 years postoperatively.
For patients undergoing a subtrochanteric osteotomy, crutch walking for 10-12 weeks is anticipated until the osteotomy is clinically healed. At this point, further rehabilitation is initiated.
Complications
Excessive limb lengthening and nerve palsy are serious potential complications of primary THA in patients with DDH. Accurate preoperative and intraoperative leg length assessment is critical. It is useful to place a leg length pin in the ischium near the desired hip center before dislocation. The pin’s axial position can be marked on the vastus lateralis with a stitch before dislocation and after reconstruction. Limb lengthening >2.5 cm should be avoided to minimize risks of femoral and sciatic nerve palsy. During surgery, limited hip flexion (20°-30°) and knee flexion (30°-40° ) minimize excess tension on the femoral and sciatic nerves. Femoral and sciatic nerve palsies occur most frequently in patients who have had an earlier surgery in which anterior or posterior scars inhibit nerve excursion during limb lengthening. Careful retractor placement must be constantly monitored. Finally, in patients undergoing limb lengthening >1.5 cm, an awake test is performed to assess sciatic and femoral nerve function. Preoperative patient instruction and anesthesia cooperation are required. Sciatic nerve-evoked potential monitoring during surgery may also be useful. Other potential complications of DDH surgery include femoral fractures and trochanteric complications. For small femoral canals, power milling is preferable to broaching, and smaller than standard instruments and implants may be needed. For short femurs or femurs shortened by subtrochanteric excisional osteotomy, perforation of the anterior femoral bow can be avoided by flexible reaming of the canal or shortening the femoral implant with a metal cutting device. Trochanteric abutment against the ischium in extension, especially in hips with a high hip center, can be minimized by a derotational subtrochanteric osteotomy or trimming the posterior trochanter and/or lateral ischium. A subtrochanteric osteotomy is preferred to a trochanteric advancement with femoral neck shortening to avoid complications with trochanteric fixation (eg, nonunion, fibrous-union, bursitis).
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Figure 9: Preoperative (A) and 5-year postoperative (B) radiographs after staged bilateral sub-trochanteric osteotomies are performed to treat dislocated (left) and fused (right) hips with plate removal through the osteotomy resection level. |
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Results
Since 1992, the author has used 120 modular stems for cementless femoral fixation in patients with DDH hips (Crowe I–Crowe IV) undergoing primary THA. Complications include three femoral nerve palsies (all resolved), one sciatic nerve palsy (not resolved), two intraoperative fractures treated with cerclage wiring, and one non-recurrent posterior dislocation. All femoral stems are stable and ingrown. Only one stem has been revised due to periprosthetic fracture at 4 years postoperatively (Figure 10).
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Figure 10: Serial radiographs demonstrate versatility of modular stems in femoral deformity and periprosthetic fractures. Preoperative radiograph demonstrates proximal deformity after femoral osteotomy (A). Postoperative radiograph shows a proximal sleeve triangle placed laterally to improve host bone contact (B). A calcar replacement neck was used to accommodate low neck cut and to equalize leg lengths. The patient sustained a transverse fracture below the stem at 2 years’ postoperatively after falling from 8-ft staging (C). The index stem has been replaced with a longer stem through the sleeve (D). A cortical onlay graft has been added. |
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References
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- Crowe JF, Mani VJ, Ranawat CS. Total hip replacement in congenital dislocation and dysplasia of the hip. J Bone Joint Surg Am. 1979; 61:15-23.
- Garvin KL, Bowen MK, Salvati EA, Ranawat CS. Long-term results of total hip arthroplasty in congenital dislocation and dysplasia of the hip. A follow-up note. J Bone Joint Surg Am. 1991; 73:1348-1354.
- Harris WH. Etiology of osteoarthritis of the hip. Clin Orthop. 1986; 213:20-33.
- MacKenzie JR, Kelley SS, Johnston RC. Total hip replacement for coxarthrosis secondary to congenital dysplasia and dislocation of the hip. Long-term results. J Bone Joint Surg Am. 1996; 78:55-61.
- Nobel PC, Alexander JW, Lindahl LJ, Yew DT, Granberry WM, Tullos HS. The anatomic basis of femoral component design. Clin Orthop. 1998; 235:148-165.
- Bobyn JD, Tanzer M, Krygier JJ, Dujoune AR, Brooks CE. Concerns with modularity in total hip arthroplasty. Clin Orthop. 1994; 298:27-36.
- Ohl MD, Whiteside LA, McCarthy DS, White SE. Torsional fixation of a modular femoral hip component. Clin Orthop. 1993; 287:135-141.
- Masonis JL, Patel JV, Miu A, et al. Subtrochanteric shortening and derotational osteotomy in primary total hip arthroplasty for patients with severe hip dysplasia. J Arthroplasty. 2003; 18:68-73.
Author
Dr Mattingly, Director of the Otto E. AuFranc fellowship, is from the New England Baptist Hospital in Boston, Mass.