The S-ROM Modular Stem for Femoral Deformities

Abstract

Many conditions produce femoral deformities that complicate primary total hip arthroplasty (THA). In each patient, treatment must be individualized to address the level of the deformity, the type of deformity, and bone quality, as well as surgeon preference. The S-ROM femoral component (DePuy Orthopaedics, Warsaw, Ind) is useful for many deformities from the level of the greater trochanter to the diaphysis. The S-ROM stem optimizes proximal and distal implant stability, allows maximal torsional stability during osteotomies, and permits easy adjustments to anteversion, offset, and leg length to provide optimal biomechanical reconstruction of the deformed femur in THA. Clinical results with few complications are achievable when using the S-ROM stem in complex deformities.

Many conditions produce femoral deformities that complicate primary total hip arthroplasty (THA).^1-5 The most common conditions are developmental dysplasia of the hip (DDH), prior surgery (eg, osteotomy), posttraumatic deformity, secondary osteoarthritis (eg, Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and sepsis), and coxa vara and coxa valga deformities. Less common conditions include a small femoral canal (eg, juvenile rheumatoid arthritis, achondroplastic dwarfism, and spondyloepiphyseal dysplasia), large femoral canals (eg, ankylosing spondylitis, rheumatoid arthritis, and alcoholic bone disease), and Paget’s disease.

Preoperative planning is important and should include a complete patient history and physical examination with emphasis on leg lengths and neurovascular status. Minimum radiograph evaluation includes anteroposterior (AP) pelvis and AP and Lauenstein lateral radiographs with a magnification marker of the hip and proximal femur. If necessary, then a computed tomography scan or scanogram may be indicated. Radiographs should identify equipment and prosthetic needs as well as the need for a bone graft or osteotomy.

Treatment must be individualized in each patient to address the level of deformity, the type of deformity, and bone quality, as well as surgeon preference. It is important for surgeons to select the proper implant and surgical approach and to be familiar with specialized techniques such as trochanteric osteotomy, corrective osteotomy, and leg lengthening.

Treatment options for femoral deformity can be managed by altering the bone to fit the prostheses (osteotomy), selecting a prostheses to fit the femur (modular stems or custom stems), or using short implants or surface replacements to avoid increased distal deformity. This article highlights the use of modular stems with or without an osteotomy for management of femoral deformity.

Levels of Deformity

Greater Trochanteric Deformity
Many conditions lead to deformity of the greater trochanter, including premature growth arrest, secondary forms of osteoarthritis, DDH, posttraumatic arthritis, and a prior proximal femoral osteotomy. Preoperative location of the greater trochanter may not allow proper instrumentation of the femoral canal and osteotomy, and advancement may be indicated for proper placement of the femoral component. Because the greater trochanter can be easily fractured during hardware removal, it is advisable to perform a controlled osteotomy and advancement to avoid nonunion, malunion, or migration. The S-ROM prosthesis can be easily used with trochanteric fixation devices, including cable grips systems (Figure 1), wires, or the S-ROM proximal trochanteric bolt.


Figure 1: Preoperative radiograph (A) demonstrating proximal femoral deformity with overgrowth of greater trochanter treated with trochanteric osteotomy and the S-ROM modular femoral implant (B).

Femoral Neck Deformity
The S-ROM stem is available in standard, +4, +6, +8, and +12 offset stems. This variety of offset stems establishes adequate myofascial tension, allowing easy management of coxa vara and coxa valga deformities, and prevents limb length inequality. The +8 or +12 offset stems are used for coxa vara deformities, whereas the standard or +4 stems are used for coxa valga deformities.

Abnormal version can also occur through the femoral neck. Modular cementless stems work well in this situation because they allow a surgeon to place the proximal sleeve in a position maximizing contact with proximal host bone and to place the stem in the position of optimal joint stability. By using cementless stems, surgeons avoid using small cemented stems with small cement mantles or suboptimal placement of unitized stems in positions that do not maximize proximal bone contact or stability. The distal flutes on the S-ROM modular stem provide rotational stability when used with a subtrochanteric derotational osteotomy to correct excessive anteversion in high-riding DDH hips (Figure 2).^4-7


Figure 2: Bilateral subtrochanteric osteotomies for high-riding DDH: preoperative (A) and postoperative (B) radiographs.

Metaphyseal Deformity
Metaphyseal deformities usually occur as a result of trauma to the proximal femur or as a result of prior surgery, such as an osteotomy.^1,3,8 Modular stems work well when the proximal sleeve is positioned in line with the deformity and the stem is placed in a position of maximum joint stability and optimal diaphyseal contact (Figure 3). Leg length can be restored by using calcar replacement necks. If a fracture nonunion occurs in the metaphyseal area, then the deformity can be resected and replaced with a calcar replacement implant with advancement of the greater trochanter. Monoblock metaphyseal filling implants work poorly in this situation due to an inadequate fit and an increased incidence of fracture.


Figure 3: Preoperative (A) and postoperative (B) radiographs showing placement of a triangular sleeve laterally to maximize proximal bone contact. Leg length was restored by using a calcar replacement stem.

Metaphyseal/Diaphyseal Deformities
Metaphyseal/diaphyseal deformities occur as a result of a developmental mismatch between the metaphysis and diaphysis, large femoral canals, small femoral canals, or a deformity from a prior surgery or trauma. Modularity is ideal for a metaphyseal/diaphyseal mismatch because 10 proximal sleeve sizes can be used with each femoral stem, allowing optimal proximal and distal fit and fill to occur independently.

S-ROM modular stems also work well in Dorr type C bone when the femoral canal is enlarged into a stovepipe deformity. Large diameter stems and oversized sleeves maximize proximal and distal contact and stability (Figure 4). The coronal slot in the stem reduces stiffness when large implants are required in patients. These large canals often occur in younger patients with rheumatoid arthritis, ankylosis spondylitis, or alcoholic bone disease.


Figure 4: Preoperative (A) and postoperative (B) radiographs demonstrating proximal and distal femoral fit and fill in an enlarged canal.

Modularity also allows easy management of small femoral canals, eliminating the need for small cemented femoral stems in thin cement mantles. Custom or off-the-shelf modular implants are available in diameters as small as 6 mm. Custom modular implants are often useful for treating patients with spondyloepiphyseal dysplasia, juvenile rheumatoid arthritis, or achondroplastic dwarfism.

Metaphyseal/diaphyseal deformities usually occur as a result of a prior surgery or osteotomy. If a subtrochanteric osteotomy is performed, then hardware can be easily removed from the osteotomy site (Figure 5). The distal flutes on the S-ROM stem provide rotational stability if an osteotomy is performed.


Figure 5: Preoperative (A) and postoperative (B) radiographs demonstrating hardware removal through subtrochanteric osteotomy for fused hip (right) and high-riding DDH hip (left).

Diaphyseal Deformity
Most diaphyseal deformities occur as a result of prior trauma. A correctional osteotomy may be required to safely place the implant and restore proper biomechanics to the involved femur and hip.^5,9 Rotational stability can be obtained in the distal fragment with the S-ROM distal flutes. Because rotational stability allows rapid healing and weight bearing, most surgeries can be performed in one stage. A two-stage osteotomy may be useful if infection is present or if preliminary hardware removal is required (Figure 6). Corrective osteotomies and placement of a long-stem S-ROM is useful in patients with Paget’s disease to prevent deformities that may result from angulation.


Figure 6: Serial radiographs showing nonunion of hip fusion and femur fracture (A) treated by removing initial selective hardware and tissue culture (B). Final reconstruction with hip fusion takedown combined with femoral osteotomy and long stem S-ROM femoral implant (C).

Summary

The S-ROM modular stem optimizes proximal and distal implant stability, allows maximal torsional stability during osteotomies, and permits easy adjustments to anteversion, offset, and leg length to provide optimal biomechanical reconstruction of the deformed femur in THA. Modular stems manage deformities from the level of the greater trochanter to the diaphyseal level. Treatment must be individualized according to the level of the deformity, the type of the deformity, bone quality, patient factors, and surgeon preferences. Clinical results with few complications are achievable when using the S-ROM modular stem in complex deformities.

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Author

Dr Mattingly, Director of the Otto E. AuFranc fellowship, is from the New England Baptist Hospital in Boston, Mass.