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Modular Femoral Component for Conversion of Previous Hip Surgery in Total Hip Arthroplasty

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Abstract

The conversion of previous hip surgery to total hip arthroplasty creates a durable construct that is anatomically accurate. Most femoral components with either cemented or cementless design have a fixed tapered proximal shape. The proximal femoral anatomy is changed due to previous hip surgery for fixation of an intertrochanteric hip fracture, proximal femoral osteotomy, or a fibular allograft for avascular necrosis. The modular S-ROM (DePuy Orthopaedics Inc., Warsaw, Ind) hip stem accommodates these issues and independently prepares the proximal and distal portion of the femur. In preparation and implantation, the S-ROM hip stem creates less hoop stresses on potentially fragile stress risers from screws and thin bone. The S-ROM hip stem also prepares a previously distorted anatomy by milling through cortical bone that can occlude the femoral medullar canals and recreate proper femoral anteversion and reduces the risk of intraoperative or postoperative periprosthetic fracture due to the flexible titanium-slotted stem. The S-ROM femoral stem is recommended for challenging total hip reconstructions.

The conversion of previous hip surgery to total hip arthroplasty (THA) can be problematic for orthopedic surgeons who try to create a durable construct and an anatomically accurate reconstruction. Restoration of anatomical relationships is necessary to produce balance and function of the muscles to reduce or eliminate limp and the risk of dislocation. Most femoral components with either a cementless or cemented design have a tapered proximal shape. The medial flare in the proximal femur distributes stresses to the bone and resists torsional stresses. This process relies on anatomy that closely corresponds to the device’s shape. The medial flare requires a cancellous bed to interdigitate cement or a bony bed to allow ingrowth into the proximal coating of the femoral component. Clinical situations exist where accomplishing this can be daunting.

Patients requiring THA present a challenge to orthopedic surgeons who must face the following issues: longer surgical times to remove the existing devices; increased blood loss while dissecting through old scar tissues; distorted proximal femur anatomy; poor bone quality as a result of preexisting osteoporosis; greater trochanter disruption, which can lead to increased rate of dislocation and difficult ambulatory function; and cement pressurization difficulties in the presence of cortical screw holes.¹

Figure 1: AP radiograph of an unstable hip fracture after loss of fixation and cut out of the lag screw. Figure 2: Lateral radiograph of an unstable hip fracture after loss of fixation and cut out of the lag screw. Figure 3: Postoperative THA with modular femoral component and strut allograft of patient in Figures 1 and 2.

Distortion of the Proximal Femoral Anatomy

The proximal femoral anatomy can be changed as a result of a fracture of the intertrochanteric region. The fixation device may also contribute to distortion of the proximal femur. In elderly patients with osteoporosis, an intertrochanteric hip fracture is often fixed with a dynamic hip screw and side plate. In a small percentage of patients, failure may occur due to loss of fixation as a result of collapse of the femoral head and neck over the hip screw. This is commonly called cut out. The resulting pain may require conversion to a THA. Zhang et al¹ reported overall failure rates after open reduction and internal fixation (ORIF) in the range of 3%-12%. Haidukewych and Berry² studied results and complications in 60 patients treated with salvage THA after failed treatment of intertrochanteric hip fracture. They found that patients had good pain relief, functional improvement, and few serious orthopedic complications with the procedure. Often, calcar-replacing and long-stem implant designs were required.

Situations similar to those described may occur as a result of fibular bone graft for avascular necrosis (AVN), subcapital fracture of the femoral neck over the fixation device, or exacerbation of preexisting osteoarthritis. In patients with unstable hip fracture after fixation, comminution of the posterior medial bone in the inter-trochanteric region and the sliding hip screw accommodates impaction and collapse of fragments between the threads of the lag screw and barrel of the side plate. The distal fragment medially displaces over the proximal segment and may externally rotate. The resulting anatomy has cortical bone of the proximal femoral neck occluding the medullary canal. In addition, the consolidation of bone around the hip screw will also cause an occlusion to the femoral canal (Figures 1-4).

Figure 4: Cortical neck occluding canal drilled out during canal preparation. Figure 5: AP radiograph of trochanteric intramedullary nail. Figure 6: Failed proximal rotation osteotomy needing THA.

Recently, an intramedullary hip nail with an entrance point through the greater trochanter became popular. Advantages to this device include a shorter incision and less blood loss, as well as shortening the moment arm between the sliding screw and the intramedullary nail as opposed to the side plate. In the event of a failure and necessitation for THA, a distortion of the anatomy occurs. The medullary canal is diverted proximally to a canal into the greater trochanter (Figure 5).

Through the consolidation or obstruction of cortical bone, broaching and preparing a bed for either a cemented or cementless femoral component will be difficult. Issues of the femoral neck sliding and medial displacement also exist. The proximal femur can be changed due to a previously existing proximal femoral osteotomy for clinical situations as reconstruction of a dysplastic hip, slipped capital femoral epiphysis, osteoarthritis, or to change the orientation of a nonunion of a femoral neck to obtain union (Figure 6), which can create similar complications with stress risers from screw holes, obstruction of the proximal medullary canal, and rotation distortion. Jimenez et al³ studied 128 THAs in patients with hip fractures treated with ORIF. All patients were treated with a cemented polished calcar-replacing stem and uncemented cup. No revisions for stem loosening at mean follow-up of 24 months were reported. They concluded that this technique was a reasonable strategy to treat failed osteosynthesis of hip fractures.

Figure 7: AP radiograph of avascular necrosis with fibular bone graft in place.

An additional cause of proximal femoral obstruction is the placement of either a vascularized or nonvascularized bone graft through the femoral neck and into the femoral head to treat AVN. This process will place cortical bone in the neck region and occlude the proximal canal (Figure 7). The preparation of the femur by broaching may not be possible and may necessitate excessive force. Berend et al⁴ followed 73 patients treated for failed vascularized fibular grafting with THA using several designs of prostheses. The investigators found improved rates of revision in hips with cementless fixation and proximally porous-coated stems, and also found that cemented stems were associated with increased loosening and higher revision rates secondary to difficulty providing adequate proximal cement mantles and lateralization of the femoral component.

Fehrle et al⁵ evaluated the technical aspects of THA in patients with tibial bone grafting of the femoral head in 13 hips with aseptic necrosis. The authors concluded that special attention must be focused on adequate graft removal, particularly in the trochanteric fossa, to prevent varus placement of the femoral component.

Preparation of the Femur

Figure 8: Postoperative AP radiograph of S-ROM demonstrating residual site of lag and cortical screw holes.

Removal of the hip screw in THA leaves a shortened proximal femur without the proximal flare of a normal anatomy and a cylindrical anatomy often obstructed by cortical bone. The initial approach in THA requires the proximal osteotomy at or slightly proximal to the lesser trochanter. Cannulation of the femur will require drilling through this cortical obstruction and then use of a broach to prepare a bed for either a regular or calcar-replacing stem. Often, it is difficult to open the proximal femur for broaching. Use of a high-speed burr facilitates this preparation to allow broaching. Zhang et al¹ studied 19 patients with THA after ORIF and found the most common complication was intraoperative fracture of the greater trochanter (seven hips) and most of these fractures occurred during reaming and broaching of the medullary canal. Because many of these patients are elderly, the canal is larger due to osteoporosis and may have had a pre-existing stovepipe canal index, which lacks a proximal flare to accommodate the stem for rotational stability.⁶ In addition, the screw and side plate create multiple medial and lateral cortical screw holes. A large lateral hole is created where the lag screw and barrel entered (Figure 8).

Using a cemented stem will give fixation in the medullary canal yet will have permanent stress risers where the cement is extruded in the screw holes. Zhang et al¹ had difficulty pressurizing the cement in the presence of multiple cortical holes. In spite of being able to achieve only suboptimal cementation, the study showed no incidences of radiographic loosening or revisions for aseptic loosening.

Figure 9: AP radiograph of hip showing distorted anatomy of proximal femur with sliding hip screw in place. Figure 10: AP radiograph of postoperative hip from Figure 9 using a large, stiff fully porous-coated, calcar-replacing stem.

The canal may be filled with a large amount of cement, creating a thick cement mantle, due to the inability to fit a cemented stem that is usually proportionally larger in the proximal portion than the distal portion. If a desire or need to bypass the stress risers exists, then the canal may require a long stem that adds complexity. A long cemented stem requires additional reaming and cement that has all the toxicity and risk of pressurizing a greater amount of marrow elements into the venous circulation. The use of a porous-coated stem to fill the cylindrical anatomy usually necessitates a large, stiff fully porous-coated stem in an osteoporotic femur (Figures 9 and 10).

Complications often occur at the point where the anterior femoral bow abuts on the tip of the stem, creating the possibility of a perforation of the cortex and either an intraoperative fracture or a late periprosthetic fracture (Figures 11 and 12), and resulting in thigh pain. The issue of malrotation of the proximal anatomy, especially in a malunion, also exists and is difficult to correct.

Modular Stem

Figure 11: AP radiograph of preoperative ORIF of unstable intertrochanteric hip fracture that subsequently went on to failure with cut out of lag screw. Figure 12: AP radiograph of patient from Figure 11 with fully porous-coated cobalt chrome stem that had an intraoperative periprosthetic fracture of the femur at the apex of the femoral bow necessitating ORIF with locking screw plate and cables.

A modular hip stem accommodates all of the issues mentioned above and creates a construct that will not compromise the fixation or subsequent reconstructed anatomy. Once the proximal osteotomy is initiated, cannulation of the proximal femur can be accomplished with gradually enlarging straight reamers in the initial drill hole. The independent preparation of the proximal and distal portions of the femur with reaming is both gentle and precise (Figures 13 and 14). This preparation does not create large hoop stresses as in broach impaction in a thin cortical tube with stress risers from previous screw holes. Modularity offers different diameters of the isthmus and intertrochanteric region, and also a subsequent assembled device that is shaped like a stovepipe with a smaller angular gradient from the distal to proximal canal.

The proximal portion is milled to create a spout into the remnant of a neck that adds additional rotation stability and can be milled in any direction or even into the greater trochanter. The milling is gentle and can prepare cortical bone that may be an obstruction in the canal (Figure 15).

When the modular stem is inserted through the proximal sleeve, the femoral neck can be rotated to an accurate restoration of the femoral anteversion. The ability to make no rotational compromise and recreate normal offset in the component adds to hip stability (Figure 16). In a nonmodular stem, the inability to recreate offset may require lengthening of the limb for soft tissue tension. If a need exists for replacement of bone proximal to the lesser trochanter, then a calcar-replacing component to add height to the implant is necessary. The current cementless and cemented calcar-replacing implants may lack this anatomical versatility; however, the S-ROM has the additional benefit of obtaining long-term durability in biological fixation. After fixation for a fracture, stress risers in the femoral canal from screw holes, as well as loss of cancellous bone proximally, occur. In a cementless hip stem, these stress risers will disappear over time, and no cement to prevent deposition of bone will remain.

Figure 13: Distal reaming the femur for the S-ROM femoral stem. Figure 14: Proximal reaming the femur for the S-ROM femoral stem. Figure 15: Milling the proximal calcar.

The S-ROM is unique. Its distal fluted stem will add rotational stability and the slotted titanium stem will collapse down in its closed pin slot (Figure 17). This collapse is less likely to be a complication if the stem is going to abut on an anterior bow of an osteoporotic femur and is less likely to perforate or cause thigh pain. If needed, the slotted titanium stem may be extended to a longer bowed stem. The issue of pressurizing bone cement is absent. The porous-coated proximal cone is prepared by a conical reamer. The spout can then be machined with the milling device to create an optimal bed with significantly superior coaptation.

When a proximally obstructive cortical bone from the inferior femoral neck or a fibular bone exists, the canal can be expanded with reamers and the cortical bone is milled and shaped to accommodate the new implant. The malrotation that is introduced by the collapse of the fracture or intentionally created, as in the case of an elective proximal femoral osteotomy, will be accommodated and reconstructed to closer match normal anatomy.

In the event of a late periprosthetic fracture, fixation is accomplished with plates or cables. It also may be done by removal of the standard stem and reintroduction of a longer bowed stem through the existing sleeve. The revision would be easier in the absence of cement in a thin osteoporotic shell of bone or a large, stiff, fully porous-coated implant in the canal.

After experiencing complications with nonmodular implants in the past while treating patients, the authors now use and recommend the S-ROM femoral stem for total hip reconstructions. S-ROM has simplified the procedure and allowed precise and reproducible pre-operative planning.

Figure 16: Adjusting modular stem to appropriate anteversion. Figure 17: Fluted titanium S-ROM stem prior to insertion and impaction.

References

1. Zhang B, Shiu KY, Wang M. Hip arthroplasty for failed internal fixation of intertrochanteric fractures. J Arthroplasty. 2004; 19:329-333.
2. Haidukewych GJ, Berry DJ. Hip arthroplasty for salvage of failed treatment of inter-trochanteric hip fractures. J Bone Joint Surg Am. 2003; 85:899-904.
3. Jimenez ML, Sferra TA, Goldstein WM, Garapati R. Total hip arthroplasty after previous hip fracture osteosynthesis. Poster presented at: Annual Meeting of American Association of Orthopaedic Surgeons; March 2005; Washington, DC.
4. Berend KR, Gunneson E, Urbaniak JR, Vail TP. Hip arthroplasty after failed free vascularized fibular grafting for osteonecrosis in young patients. J Arthroplasty. 2003; 18:411-419.
5. Fehrle MJ, Callaghan JJ, Clark CR, Peterson KK. Uncemented total hip arthroplasty in patients with aseptic necrosis of the femoral head and previous bone grafting. J Arthroplasty. 1993; 8:1-6.
6. Noble PC, Alexander JW, Lindahl LJ, et al. The anatomic basis of femoral component design. Clin Orthop. 1988; 235:148-165.

Authors

Dr Goldstein and Ms Branson are from the Illinois Bone and Joint Institute, University of Illinois at Chicago, Morton Grove, Ill.