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Feb
21

Advances in Knee Arthroplasty for Younger Patients: Traditio

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2009-02-21 17:09:44

Total knee arthroplasty (TKA) was a remarkable development in orthopedic surgery. Joint arthroplasty and arthroscopy were perhaps the greatest innovations in orthopedics in the 20th century and occurred without the advantages of today's technology. Initially, TKA was performed only on elderly patients and those with advanced rheumatoid arthritis because of concerns with long-term wear of polyethylene. Surgeons strongly discouraged this surgery for patients younger than age 60 years because both patients and many orthopedic surgeons believed that knee implants would last only for approximately 10 years, particularly in younger and more active patients. Reports in the late 1980s and early 1990s about accelerated polyethylene wear and osteolysis substantiated the conviction that TKA was contraindicated in younger patients. This led to complacency toward TKA, thus inhibiting technological advances in the procedure to develop implants for younger and more active patients,Despite discouraging results from some studies, others reported implant survivorship without significant wear well into the second and third decade after implantation.1,2 In the early to mid-1990s, manufacturers began to change methods of sterilization when a strong correlation was discovered between implant wear and oxidation of polyethylene sterilized by gamma irradiation in air.3,4 With the change in sterilization methods and use of newer materials such as highly cross-linked polyethylene and oxinium, clinical and laboratory studies revealed a dramatic reduction in wear.5 Additionally, the continued success in pain relief with this procedure created a growing interest in total knee arthroplasty (TKA). Orthopedic surgeons began to expand the indications for this surgery in their clinical practices to younger and more active patients,In most patients, pain relief after knee arthroplasty is good, but restoration of function continues to be a concern to orthopaedic surgeons. As early as 1982, Andriacchi et al6 reported on the alterations in stair-climbing ability following TKA caused by a forward lean of the body, decreased range of motion (ROM) on stairs, and decreased velocity in descending stairs. Studies of gait found abnormalities that included decreased gait velocity and stride length, reduced midstance knee flexion and flexion during swing phase, and abnormal flexion-extension moments.

Noble et al9 reported limitations in functional activities such as moving laterally, turning, carrying loads, playing tennis, and gardening in 52% of patients after TKA compared with 22% in age-matched patients who had not undergone TKA. Huch et al10 reported reduced participation in sports activities after TKA, despite a marked reduction in knee pain. These limitations in functional activities were related to alterations in knee kinematics inherent in conventional TKA.

The first major attempt to improve knee kinematics occurred in the early 1980s with the introduction of posterior-stabilized implants. This change in implant design improved knee flexion. Since the posterior stabilized implants were introduced, few design changes were made that attempted to improve knee kinematics until recently with the Journey knee (Smith & Nephew; Memphis). The Journey knee substitutes for both the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) with cam and post mechanisms,Knee kinematics after TKA can be measured and compared with the kinematics of the normal knee. Video fluoroscopy with image-fitting techniques and custom software enables researchers to accurately measure parameters such as femoral rollback (anteroposterior translation of the femur relative to the tibia) and axial rotation as the knee flexes. A study of traditional TKA by Dennis et al11 confirmed that rollback and axial rotation are dramatically reduced with both cruciate-retaining and cruciate-substituting designs. Price et al12 documented the reduction of the patellar tendon angle (in stance) secondary to the absence of ACL function. Additional studies confirm that these alterations are much less pronounced with bicruciate-retaining knees6,13-15 and unicondylar arthroplasties.

Altered knee kinematics with traditional TKA are caused not only by sacrifice of the cruciate ligaments, but also by changes in the patient’s hard tissue anatomy. Traditional bone cuts are made independent of the soft tissues that provide stability and dictate the kinetics of knee flexion. Instruments for performing TKA were designed for patients with an average bony anatomy. However, the bony anatomy varies among patients; on average, distal femoral valgus is 9° but ranges from 5° to 12°. Femoral alignment guides are designed to create a distal femoral resection only at 5° or 6° of valgus to the anatomic axis of the femur. Therefore, the resection in a knee with 5° of distal femoral valgus requires removal of a similar amount of bone from both the distal femoral condyles, whereas a resection in a femur with 10° or 12° of valgus requires removal of considerably more bone from the medial femoral condyle. Removing differing amounts of bone from the condyles creates ligament imbalance. Similarly, removing more bone from the back of the femoral condyle compared with the end of the condyle creates flexion instability. The change in hard tissue anatomy with conventional TKA by necessity also affects the soft tissue balance and alters knee stability and kinematics,Total knee arthroplasty for younger and more active individuals requires dramatic changes in the approach to TKA, especially for high-performance knees that will permit patients to participate in athletic activities including running and jumping sports. The goal is to create an implant for TKA that performs as a normal knee. To accomplish this, new implant systems must be designed with several prerequisites. First, surgeons must acknowledge that each patient’s anatomy is unique and think in terms of customizing the instruments and implant to the individual. Second, surgeons must recognize that the relationship between the patient’s hard tissue and soft tissue is idiosyncratic. Altering one aspect must preserve the relationship with the other for an optimal outcome. Finally, surgeons must have implants that truly resurface the bone and replicate the individual patient’s anatomy,Implants of the future will be dramatically different from those currently used. In the future, considerably less bone will be removed at the time of the arthroplasty. Presently, approximately 20 mm of cartilage and bone is removed to accommodate a femoral component thickness of 9 mm and a tibial component thickness of 10 mm. The femoral component of traditional implants must be 9 mm thick to maintain enough strength to prevent implant fracture at the chamfer junctions created by planar bone cuts. By eliminating chamfer cuts, adequate strength can be maintained with a femoral component thickness of 4 to 5 mm.

Performing a full TKA on patients with degenerative arthritis isolated to only one compartment will become outdated. Younger and more active patients will choose selective resurfacing only in the areas of the joint with significant degenerative changes. Modular implants and integrated instruments will allow the surgeon to resurface all or part of the knee while preserving all ligamentous structures. Patients with isolated medial compartment arthritis will undergo unicondylar implantation, and those with isolated patellofemoral disease will benefit from patellofemoral arthroplasty. If disease progresses in the other compartments, a combination of modular implants will allow selective resurfacing without removing stable, well-functioning components,Instruments of the future also will be much different. Methods for customizing bone preparation are being developed that will allow preservation and proper tensioning of the capsule and ligaments. The most promising of these methods are tissue-guided surgery and robotic bone resections. Tissue-guided surgery uses the synergy between bone and soft tissues to guide bone preparation. The surgery can be performed using a minimally invasive approach. In knee arthroplasty, tissue-guided surgery is carried out by mounting milling instruments on the resected surfaces of the proximal tibia (Figure 2). As the knee is moved through a functional ROM, the milling instrument is activated, and the bone is removed from the femoral condyles. The depth of bone resection is controlled by placing shims beneath the cutters to increase the thickness of bone removed. The amount of bone removed is determined by tension in the patient’s soft tissue structures. Alternately, bone can be removed by load control. With this method, pressure-sensitive devices to bias the cutters control the thickness of bone removed.

Robotic bone preparation requires preoperative computed tomography (CT) imaging of the hard tissues of the knee. The robotic instruments are free standing from the femur. Next, the end of the femur is sculpted to create the optimal bone resections and the best position for the femoral component. Implants can be customized to the patient’s anatomy as determined by the preoperative CT images.

Implants of the future will be designs that truly resurface the knee, because the advanced methods of bone preparation eliminate the need for chamfer cuts. The implants will be modular and may be either aligned to each other or joined within the confines of the joint. Methods of component fixation also will change from that of bone cement. These methods will include bioactive surfaces for bone ingrowth and bonding agents with enhanced adhesive properties.






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