Orthoses designed for paralysis are specialized medical devices used to address partial or complete paralysis, as well as the complete functional failure or incomplete paralysis (paresis) of muscles or muscle groups. Their primary purpose is to correct or improve functional limitations and to replace functions that have been lost due to paralysis. These custom-made devices are crucial in helping individuals regain security and mobility, often compensating
for functional leg length differences caused by paralysis.
Customization and Material Innovation
The effectiveness and quality of a paralysis orthosis heavily depend on its fit. An optimal fit is achieved when the orthotic shell is in total contact with the patient's leg, which is why custom-made orthoses are frequently preferred. The production of a custom-made orthosis also allows for the precise integration of orthotic joints. This ensures that the dynamics of the orthosis align exactly with the pivot points of the patient's anatomical joints, allowing the orthosis to function where dictated by the patient's natural anatomy.
Reducing the weight of an orthosis is a significant factor, as it substantially lessens the energy required for a patient to walk. Consequently, the use of lightweight and highly resilient materials such as carbon fiber, titanium, and aluminum is essential in the manufacture of custom-made orthoses. These materials contribute to the stability and torsion-resistance of the orthotic shells, which are necessary for the device's overall quality and function, providing the stability needed to regain security in standing and walking.
Functional Elements for Specific Muscle Weaknesses
Orthoses can be individually configured through the use of various orthosis joints and adjustable functional elements. This adjustability allows for compensation of existing functional deviations resulting from muscle weakness. The goal of a high-quality orthotic fitting is to precisely adjust these functional elements to provide necessary support while restricting the dynamics of the lower extremities as little as possible, thereby preserving any remaining muscle functionality.
The type of orthosis, such as an Ankle-Foot Orthosis (AFO) or Knee-Ankle-Foot Orthosis (KAFO), and the specific functional elements required are determined by the strength levels of different muscle groups. For instance, if the dorsiflexors are weak, an orthosis should lift the forefoot during the swing phase to reduce the risk of stumbling. A drop foot orthosis, which has only one functional element for lifting the forefoot, is suitable for this specific weakness. However, if other muscle groups are also weak, additional functional elements are necessary. Adjustable resistance for plantar flexion can be included to adapt the passive lowering of the forefoot to the eccentric work of the dorsal flexors during the loading response.
In cases of weak plantar flexors, the orthosis must transfer significant forces that the strong muscle group would normally handle. Dynamic functional elements are preferred for the ankle joint, as static elements would completely block dorsiflexion, leading to increased energy consumption during walking. The resistance of these functional elements against unwanted dorsiflexion should be adjustable to match the degree of plantar flexor weakness. For very weak plantar flexors, high resistance is required to compensate for functional deviations.
Addressing Knee and Hip Extensor Paralysis
When knee extensors or hip extensors are weak, the orthosis must provide stability and control during the stance phase of walking. Different knee-securing functional elements are employed based on the severity of muscle weakness. For slight weakness, a free-moving mechanical knee joint with its pivot point positioned behind the anatomical knee pivot point may suffice to increase safety. However, in cases of significant weakness, knee flexion during walking must be controlled by functional elements that mechanically secure the knee joint during the early stance phases, specifically between loading response and mid-stance.
Stance phase control knee joints are designed to lock the knee during the early stance phases and release it for knee flexion during the swing phase, allowing for a more natural gait pattern despite mechanical securing against unwanted knee flexion. In contrast, locked knee joints, while offering good safety, keep the knee mechanically locked throughout the swing phase. Patients using locked knee joints must manage the swing phase with a stiff leg, often resorting to compensatory mechanisms like raising the body's center of gravity (Duchenne limping) or swinging the leg in a circular arc (circumduction). These incorrect gait patterns can lead to secondary issues in the bone and muscle system and increase energy consumption. Both stance phase control and locked knee joints can be mechanically "unlocked" to allow the user to sit down.
KAFOs for Comprehensive Support
A Knee-Ankle-Foot Orthosis (KAFO) is used when there is weakness in the knee or hip extensors, often in patients with paraplegia due to spinal cord injury, poliomyelitis, or multiple sclerosis. KAFOs typically feature two orthotic joints: an ankle joint between the foot and lower leg shells, and a knee joint between the lower leg and thigh shells. KAFOs can be categorized into three main variants: those with locked, unlocked, or locked and unlocked knee joints.
KAFOs with a locked knee joint provide necessary stability by keeping the mechanical knee joint locked during both standing and walking. While secure, this design makes it difficult to swing the leg forward, often requiring compensatory movements. KAFOs with an unlocked knee joint allow free movement but offer only minor compensation for paralysis-related issues, making them suitable only for minor paralysis of the knee and hip extensors. The most advanced KAFOs feature a mechanical knee joint that is locked during the stance phase for stability and automatically unlocked during the swing phase, allowing for approximately 60 degrees of knee flexion. This design enables a more efficient and natural gait without stumbling or compensatory mechanisms. Early developments in automatic knee joints emerged in the 1990s, and today, electromechanical and electrohydraulic systems enhance safety and comfort. These advanced KAFOs can integrate various functional elements to compensate for weaknesses in dorsiflexors or plantar flexors, with adjustable resistances for each, offering comprehensive support tailored to the user's limitations.
