By Paul T. Webber, CO, FAAOP, BOCO/P, ABDA
Currently it is estimated that there are 21 million Americans who suffer from degenerative osteoarthritis (OA). In the next 20 years, the incidence of OA will increase almost 50%. As the American Baby Boomer population ages and the enjoyment of athletic activities increases, the alternatives to early surgical intervention need to be explored. Patients who are younger than 65 are usually classified as too young for total knee replacement. Obese and medically compromised patients—regardless of age—are also ideal candidates for the use of a mechanical hinge type of OA bracing. Unilateral Hinge Studies in the United States and Canada have proven the effectiveness of the design of an OA orthosis that relies on the use of a unilateral hinge; the hinge’s three-point pressure design factor either accommodates or provides some correction of the varus or valgus deformity. A force is applied to the lateral aspect of the thigh and calf and then countered by a laterally directed force from a dynamic force strap that wraps around the medial aspect of the knee and the lateral hinge and pulls the knee out of valgus at extension during the stance phase of gait. This is an example of a valgus-correcting type of orthosis. There are other types of OA orthosis that use a laterally directed force applied with a “push” rather than a “pull” when dealing with valgus correction or accommodation. When the lateral compartment of the knee is compromised, for example, with a valgus type deformity, there is a subsequent increase in the pain and pressure on the lateral knee compartment. If left unsupported, this will continue to progress to where the only viable option would be surgical intervention. To delay this progression, the use of an orthosis that will unload the lateral compartment would be an appropriate choice. Fortunately, this is also one of the easiest choices to accommodate with the most common unilateral type of OA orthosis. The polyaxial hinge is placed on the lateral side of the affected leg, and a dynamic force strap that wraps around the medial aspect of the knee uses the lateral placed hinge to apply a varus moment during ambulation. This works well with most patients suffering from a lateral compartment impingement as the lateral hinge allows for clearance with the contralateral limb during the swing through and stance phases of gait. This design also allows for protection of the medial collateral ligament and anterior cruciate ligament (ACL) as a rigid shell design helps to prevent anterior tibial translation. The market is full of different types of unilateral hinge OA orthoses. A design consideration of these types of orthosis depends on the problem that needs to be addressed. The availability of a design that tracks the joint through its movements during ambulation is the most important consideration. A mechanical knee joint that by design cannot follow the knee during flexion puts untenable forces on a compromised knee joint. A mechanical knee joint that can be adjusted to provide various degrees of correction in either the needed varus or valgus moments at heel strike provides a continuous means of verifying a patient’s progress. A polycentric or genu-centric mechanical knee joint is the single most important determinant of the appropriateness of an OA orthosis. The flexibility of the foot and ankle complex must be addressed at this stage as well. If the patient is unable to counter the valgus correction of the orthosis with the ability to externally rotate the talocrural joint to allow for a more appropriate foot and ankle alignment, then the use of custom orthotics should be looked into as well. Conversely, when an internal rotation of the talocrural joint is needed to counter the varus moment of the knee at heel strike, a lateral wedge can be placed in the patient’s shoe to help properly align the foot and ankle complex. Physical therapy is another venue of rehabilitation that is vital to producing better outcomes. The proper gait training that is offered by a qualified PT can lead to far better outcomes. Occupational therapy can be used for the teaching of proper techniques for activities of daily living (ADL) functions. These precautionary steps of using ancillary providers with a team approach to OA management lead to better outcomes and long-term financial benefits as well. Bilateral Hinge The use of a bilateral hinge type of knee orthosis (KO) is another alternative. The design considerations are similar to the needs for a unilateral hinge. The need for a mechanical hinge that can track the movements of the knee during flexion and extension is paramount. The pathological movements of a knee affected by osteoarthritis have to be accommodated or corrected, if possible, by the mechanical hinge on the KO. Alignment deformities on the weight-bearing surfaces in the knee tend to be progressive. The progressive varus deformity associated with medial compartment arthritis leads to further medialization of the majority of the weight-bearing forces. This is even more accentuated as time progresses. The weight of the patient is also a consideration; heavier patients tend to have problems with the fit and function of OA orthoses in general. Focusing on a varus deformity or a medial impinged compartment, forces must be applied to the medial thigh and calf while a counterforce is applied by the placement of the knee joint itself by using condylar pads. The associated deterioration of the collateral ligaments as well as the cruciate ligaments again needs to be addressed. One available design features a 20Ž varus or valgus range of adjustment. This is accomplished by the use of a “biaxial” hinge. In our example of a varus deformity, the lateral upright is adjusted through the use of a “cam” type device on the upper and lower uprights that applies a corrective force on the knee itself. Soft condylar pads are used, and the knee is cushioned between these pads to preclude any shifting of the applied load. The ability to track changes in the affected limb is an important adjunct to the improved function of the arthritic knee. By tracking changes that are made over time, the correction gained can be physically measured to substantiate outcomes measurements. A KO that is designed to protect from or prevent ACL injuries can be used with success as well. A custom-made KO with a genu-centric hinge that is preloaded to correct the varus/valgus deformity during measurement and fabrication is also beneficial to the patient. This type of device can be used when, during the measurement or casting for the orthosis itself, the knee joint can be placed in an easily corrected position. During the evaluation for a KO, the knee is flexed and extended in its current position and then a medial or lateral force is applied to correct either the varus or valgus deformity and the limb is flexed and extended again. The patient should be able to note the changes in the proprioception of the movement of the knee, and determine which is a more comfortable range of motion. The affected limb can then be either casted or measured and a custom-fabricated KO can be made to those measurements. Custom Fabrication VS Custom Fit With the prevalence of managed care types of insurance plans, payors increasingly desire money-saving alternatives. They require a statement of medical necessity to show that a less costly alternative is not appropriate. This is where the decision to provide a custom-fit or a custom-fabricated device must be made. The physician has made a decision to delay surgical intervention. The next step is to supply a mechanical device, an orthosis, that will provide appropriate support and relief from the pain of osteoarthritis. A custom-fit orthosis, one that is sized extra small to extra large, for example, is designed for legs that fit a statistical norm for a given size. Every manufacturer of these custom-fit devices has its own sizing chart. This is usually either a circumference measurement of a specific site or an anteroposterior measurement. A qualified and trained orthotist provides the experience and education to measure, fit and adjust a custom-fit KO. Custom-fit OA orthoses are usually cheaper to furnish initially to a patient suffering from the early effects of osteoarthritis. If a patient’s leg fits the design parameters, then a trial fitting is worthwhile. Usually the patient can tell if the orthosis can be worn comfortably. During the evaluation of the fit and function, the orthotist can determine whether wearing the orthosis will provide any relief. Contraindications for the use of a custom-fit orthosis would include the patient that does not fit into the statistical norm, or a knee that is unable to conform to the mechanical limitations of off-the-shelf design. Ligament laxity that is unable to be supported by the custom-fit design would be a contraindication as well. The biggest complaint about custom fit designed orthoses is that the suspension of the device can be spotty. There is a tendency to migrate distally, or rotate on the affected limb, causing discomfort especially at the tibial crest and the condyles. Custom-fit orthoses are just that—custom fit to a statistical norm; whether or not that is close enough to provide the support and correction needed is best determined by an evaluation by the physician or orthotist. That said, the option of custom fabrication is appropriate when a problem exists with either the skeletal anatomy or the progressive degeneration of the anatomical knee joint. The need for ligamentous stability that cannot be accommodated with custom-fit designs is also an indicator of the need for a custom-fabricated type of OA orthosis. Patients with a large or disproportionate thigh or small calf circumference rarely will fit into the custom-fit model. Women especially are hard to fit with a made-to-measure device. The taper of the thigh and a small calf usually will preclude the use of an off-the-shelf type of orthosis. These two factors are the main reason for the tendency for an orthosis to migrate distally. The ability to design an orthosis that fits the skeletal anatomy as perfectly as possible is another reason for the desire for custom fabrication. There is no off-the-shelf device that can fit with as close a tolerance as a device that is made directly to a patient model. This is often necessary when dealing with OA, as the forces that are needed to accommodate or correct a mal-aligned anatomical knee are great. To distribute the forces with as much accuracy and comfort as possible, the orthosis should be made from a patient model. Often a knee that is afflicted with OA will also demonstrate ligamentous laxity in that joint. These weakened ligaments need to be supported and protected, especially if the need for joint replacement looms in the not too distant future. N Paul T. Webber, CO, FAAOP, BOCO/P, ABDA, is a contributing writer for Rehab Management.
