Until recently, people who suffered weakness in the quadriceps muscle (which keeps the knee from collapsing), as a result of stroke, diabetes, spinal cord injury, polio, multiple sclerosis, femoral nerve palsy, or lumbar plexopathy, were relegated to the use of heavy, bulky leg braces that did not bend at the knee joint. The result was a stiff-legged step that was miles away from natural walking. In fact, simply trying to clear the floor with a straight leg was a chore; even sitting with such a brace was a grueling task.

Of course, there was another option, as other braces allowed the knee to stay unlocked, enabling the leg to swing more easily. Unfortunately, the unlocked knee joint—also known as the "free knee" joint—was incapable of preventing the knee from buckling; ironically, this is often the very reason a person may have needed this kind of brace in the first place.¹

A slight improvement was the development of drop locks or bail locks attached to the brace, which would allow a person to stand and lock the knee in full extension. In addition, these locks could be manually activated to allow knee flexion for sitting and other purposes, but these were not intended to be actuated while walking. Consequently, the patient would be forced to ambulate with an awkward gait pattern. For some, the sheer weight of the brace proved impossible to lift; for others, limited motion was the best they could hope for.

Then, about 4 years ago, a new brace was developed that would provide sufficient support to the knee joint yet would bend at the swing phase of a person's walk to enable normal ambulation. Enter the stance phase lock (SPL) brace. A long-awaited improvement to standard bracing, using technology originally developed by NASA, the SPL brace is designed to lock the knee joint at the moment of heel strike (at which time, incidentally, the heel must endure twice the person's body weight). This locking mechanism ensures that the knee does not collapse and send the person to the floor at heel strike, the initial weight-bearing phase of the step and the second phase of the step after the swing. This locking function is intended to actually replace the extensor function of the human body, a function that is often lost because of a stroke or other neurological incident.

As the knee becomes unweighted through toe-off (the last phase of a person's step after flat foot), the locking mechanism releases the knee into a free swing, while the extensors keep the person erect. The SPL brace helps create a "clean" swing and a near-normal cadence that replaces the stiff-legged, rigid gait generated by traditional braces.

Ultimately, the SPL is suitable for individuals with the following indications²:

Isolated quad weakness/absence;
Unilateral leg paralysis/paresis;
Increased stability for floor reaction (FR) AFO candidates;
Increased stability for offset knee KAFO (knee-ankle-foot orthotics) wearers;
Increased stability for free knee KAFO wearers; and
Increased stability for solid ankle/PF (plastic ankle foot orthosis) stop AFO wearers.

The locking mechanism in the SPL brace is housed in the outside knee joint and works through a combination of gravity and muscle power. Gravity affects this particular knee joint through a pendulum-type design; the pendulum tips forward and back (depending on where the knee joint is positioned in space) when the knee is in front of the person while stepping forward. The pendulum slips back, locking the knee, at full extension. If the leg is behind the person, the pendulum slips forward as he or she prepares to take a step. A slight knee hyperextension thrust and this pendulum forward position unlocks the knee to allow free swing. What is more, the SPL knee is completely independent of the foot and ankle, not requiring any motion in this joint to function.¹

Interestingly, the appearance of SPL braces has necessitated the corresponding development of new mechanical endurance testing methods to ensure the safety of these devices. Specifically, the lock/release mechanisms, dynamic range of joint motion, and, in some cases, the capability of locking at different joint positions, while highly desirable benefits of the SPL, are the very reasons this enhanced testing has been cultivated. The result is a new mechanical fatigue tester, as reported by the American Academy of Orthotists and Prosthetists.³

A more in-depth understanding of the workings of the knee joint will create a greater appreciation of the SPL's remarkable function. The knee joint does not have a distinct center of rotation. As we flex and extend, the femur rides atop the tibia as if on a slippery surface, bending and shifting in an anterior and posterior fashion. The patella (knee cap) also rides along imbedded in the quadriceps tendon. The bones above and below the patella are encased in a network of tendons and ligaments that work very hard to hold all in a neat and congruent formation. The position of the patella on the anterior of the knee joint gives added strength for extension of the quadriceps. This mechanism is very important for standing and propulsion. The bottom line is that, to be effective, a brace such as the SPL must work seamlessly in this highly developed and specialized system.

One might wonder why such a revolutionary brace took so long to develop. The truth is that prosthetics, which is closely linked to orthotics, has traditionally been far ahead of the orthotics arena. In prosthetics, you can put a piece of anything under a stump, then attach the individual components. When you put on a brace, it is outside the body. Instead of being a replacement part, it is an adjunct item. Consequently, it must work in conjunction with an existing body piece.

The fact is, the need for devices such as the SPL will increase as the American Baby Boomer population continues to age. This generation, by and large, enjoys an active lifestyle and wants to maintain their mobility as long as possible. A brace such as the SPL allows a person with quad weakness to do so with a very natural, energy-efficient gait that is much less exhausting to perform than the stiff-legged walk produced by previous braces.

Fortunately, recent developments such as the SPL are helping brace manufacturers keep up with the demands of today's population. These new knee joints fitted by a certified and qualified orthotist can enhance a person's recovery, and facilitate a positive return to the community.

John J. Griffin, CPO, BOCOP, RTP, is the director of the Orthotics and Prosthetics Department at HealthSouth Braintree Rehabilitation Hospital, Braintree, Mass. He is a member of the American Academy of Orthotists and Prosthetists (AAOP) and is certified by the American Board for Certification in Orthotics and Prosthetics.

REFERENCES

1. Major technological advances in knee-ankle-foot (KAFOs) orthoses. Available at: www.whnt.com/Global/story.asp?S=3473828&nav=menu108_10 Major Technological Advances in Knee-Ankle-Foot Orthoses. Accessed May 24, 2006.
2. Horton achieves technology breakthrough. Available at: www.oandp.com/edge/issues/articles/2002-05_06.asp. Accessed May 24, 2006.
3. Fatigue test device for stance phase control knee orthoses. Available at: www.oandp.org/jpo/library/2003_04_143.asp. Accessed May 24, 2006.