by H. Thomas Temple, MD
H. Thomas Temple, MD, is associate professor, orthopaedic surgery, University of Virginia Health Sciences Center, Charlottesville, Va.
Surgeons performing limb salvage following resection for high grade osteosarcoma of the distal end of the femur must understand tumor biology, treatment options, and the needs and desires of the patient. Despite significant advances in treating osteosarcoma, survival in patients with lesions in the distal femur is about 50 percent.1 The orthopaedist's primary goal is to achieve local disease control and to maximize function in consonance with the patient's lifestyle and needs. Since amputation is the standard by which salvage strategies must be compared, outcomes measured in local disease control and, more importantly, survival, must be the same for limb salvage with the potential added benefits of improved function, patient satisfaction and cost.
One multi-institutional study1 has shown that although local disease control is better in patients treated by amputation vs. limb salvage, survival is the same; in addition, return to work is more likely and function is better in limb salvage patients.
There are many limb salvage techniques following tumor resection, although following resection for high grade osteosarcomas of the distal femur, most surgeons choose metal prostheses rather than allografts or allograft composites. It is important to recognize that one is not necessarily better than another, but under certain circumstances and in a particular institution's experience, one may be preferred. An ideal reconstruction should provide a painless, stable and immediately functional result that is durable. It should be immediately functional because half of these patients will succumb to their disease within five years and durable because the other half will survive. Metal reconstructions fulfill these criteria in most cases. Furthermore, metal constructs can be used to serially lengthen an extremity in a young patient as well as satisfy defects following extra-articular resection; two conditions that are difficult to address with biologic alternatives.
In large series of patients treated with segmental prosthetic reconstruction, revision rates, incidence of infection and overall function are satisfactory in most cases1-6, 8, 9 Unwin8 analyzed 1,001 cases, of which 493 were in the distal femur, demonstrating prosthetic survival of 67.4 percent at 10 years, while Ward,9 in 48 patients with distal femoral endoprosthetic replacements reported 83 percent survival at 71 months. Using modular segmental devices only, Henshaw4 reported 95 percent survivorship in 44 distal femoral endoprosthetic replacements since 1990 with average follow-up of 37 months. Infection rates in all series were less than 10 percent,2,3,6,9 and there is no risk of viral disease transmission.
Major advances in metal reconstruction include modularity; improved knee designs, specifically, the rotating hinge; and better fixation to host bone, i.e., porous coating at the host metal junction and extramedullary fixation. Unlike biologic alternatives, availability, retrieval, storage and handling are not problematic.
Modularity has resulted in increased availability and decreased cost. Early metal designs were custom fabricated, resulting in delays between diagnosis and reconstruction; in addition, intraoperative flexibility was limited. Tumors change with treatment and unforeseen equipment and design problems occur, all mitigated in whole or part by modularity. Current modular systems employ Morse tapers with varying segment lengths and stem diameters to ensure adequate fit and fill but despite multiple junctions, metal fatigue failures and component dissociation are less than 1 percent.7 Biologic healing to bone is not necessary, thus enabling patients to mobilize soon after surgery.
Although early and late periprosthetic fractures are rare, aseptic loosening remains the most common cause of failure. Design modifications such as larger diameter stems, greater radii of curvature at the stem bases and the addition of porous coating at the prosthesis bone junction further reduce stem failures and aseptic loosening.5,7,9 Extracortical bone bypasses force around the stem to the adjacent cortex resulting in decreased shear stress at the prosthetic and bone-cement interfaces and less stress shielding. Furthermore, circumferential bone around the porous coating creates a "biologic purse-string"10 thus minimizing canal access to wear particles. Periprosthetic wear and bone loss are diminished by current prosthetic designs that employ multiple degrees of freedom at the knee articulation, resulting in decreased tibial stress.
With modern implants, tibial loosening is not as significant a problem in even young and active patients as it was with older constrained devices. Wear is most frequently seen in the inner bushings where force is concentrated, however, worn components can be easily exchanged.2 Finally, surgical improvements to include adequate canal reaming to accommodate larger stem diameters, third generation cement techniques, and meticulous soft tissue handling to provide good prosthetic coverage and balance about the knee have improved long-term component survival.
Although there is no perfect solution to the problem of large ostearticular defects following tumor resection, continued advances in prosthetic design, prudence in patient selection and attention to surgical detail will result in even better and more durable function in the future.
A design concept - "the biologic purse string." First National Conference on the Use of The Modular Replacement System, Chicago, Ill., November 1996.