Role of Modular Designs in Complex THA
In the abstract of his presentation from ICJR’s Pan Pacific Orthopaedic Congress, Dr. Robert Barrack discusses the stem designs introduced to address challenging revision total hip arthroplasties.
By Robert L. Barrack, MD
As with primary cementless total hip arthroplasty (THA), the dominant cementless revision stem design concept was an extensively coated, cylindrical, cobalt-chrome (CoCr) stem with a collar. The same basic surgical technique involved obtaining a scratch fit 4 to 6 cm into the diaphysis.
It became apparent that with stem length greater than 7 inches, a bowed stem was frequently desirable to conform with the bow of the native femur and lower the risk of perioperative fracture. This approach proved to be relatively successful and dominated the American market.
As the volume and severity of revision procedures continued to increase, a number of issues were identified and revision stem design evolved. Among the clinical problems that were identified included:
- The relatively high incidence of perioperative fracture, thigh pain and stress shielding associated with large, stiff, CoCr stems
- Difficulty consistently obtaining mechanical stability with higher degrees of bone loss
- Instability and limb length issues partially attributable to the limitations of adjusting length and offset with a solid, one-piece stem with fixed angle of neck version relative to the shaft
Many of these issues have been addressed with the introduction of conical, titanium, modular revision stems. These stems were initially utilized in complex revision cases in which a solid fully coated stem presented challenges.
The clinical results and surgical technique proved to be so desirable, however, that module stem designs became the dominant approach to femoral revision at many centers.
The goals of revision THA are no different than those of primary THA: restore biomechanics and reduce or eliminate pain with a durable construct that should last the life of the patient.
With this in mind, the first step should be to establish where fixation will be achieved. This, of course, depends on the degree of bone loss after removal of the primary implant, which is not always fully known or appreciated from preoperative imaging. Similarly bone quality may not be found to be as good as it may have seemed from the preoperative imaging. Therefore the right revision femoral component may not be identifiable prior to the operation. This has led to the dominance of modular systems in revision THA.
These systems typically offer several stem and body types that allow for fixation in different parts of the femur and that can be chosen during the operation. Once fixation is achieved, attention must be turned to the biomechanics. The ability to adjust leg length, offset, and anteversion independently allows the surgeon to optimize the reconstruction.
Care must be taken with anteversion (which is not commonly discussed), as adjusting the anteversion effectively translates the femur anteriorly or posteriorly within the soft tissue envelope of the femur and, thus, can have a marked effect on the forces being conducted by the reconstruction to the bone. Exact reconstruction of leg length, offset, and anteversion will result in a more natural feeling joint and lower forces being transmitted to the bone interfaces and to the bearing.
The “just in case” factor is another important consideration in terms of implant design. The surgeon may be unhappy with result at final trial reduction, and spontaneous bony fractures such as trochanteric escape are always a potential risk. Therefore, the ideal system is easy to use with plates, grips, and cables, and it can also be disassembled in situ without having to remove a well-fixed distal stem.
Finally, the ease of use of the instruments and the track record of the implant itself must be considered, as these can considerably reduce stress within the OR team, which in itself can lead to a shorter, more successful operation. Following this thought process should allow the surgeon to select the right implant for the patient, himself or herself, and the OR team.
Robert L. Barrack, MD, is from Washington University School of Medicine, St. Louis, Missouri
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