0196-6011 /86/0706-0304$02.00/0 THE JOURNAL OF ORTHOPAEDIC AND SPORTS PHYSICAL THERAPY Copyr~ght 8 1986 by The Orthopaedic and Sports Physical Therapy Sections of the American Physical Therapy Association Clinical Use of the Johnson Anti-Shear Device: How and Why to Use It TERRY MALONE, EdD, PT, ATC* The purpose of this paper is to describe the clinical utilization of the Johnson Anti- Shear Device. This device presents a method of controlling the anterior shear forces developed during isokinetic exercise on a CybePll or Orthotrone system. The antishear device allows the therapist to alter the amount of anterior shear developed during exercise, thus allowing the therapist to individualize the rehabilitation protocol. This is of special significance following anterior cruciate injury. The clinical use of this device will allow the therapist to more safely develop quadriceps exercise programs with their anterior cruciate deficient knee patients. The Johnson Anti-Shear Device was introduced by the Cybexm Corporation in 1983. This device wasdeveloped through the investigative efforts of Mr. David Johnson who published the first article regarding this issue.' In this article, Johnson emphasizes that the typical application of resistance at a point distal on the extremity allows considerable shear force at the knee. This shear force may result in abnormal anterior translation of the tibia on the femur during strong quadriceps contraction. This may result in abnormal Cybex II tracings as shown in Figure 1. This graph represents what this author has termed an active anterior drawer caused by the anterior translatory forces combined with inadequate stabilization because of an incompetent anterior cruciate ligament. For this anterior shift to occur, the anterior medial band of the anterior cruciate ligament and secondary capsular stabilizers have most likely been damaged. Rather than spending a great deal of time describing how the antishear device functions biomechanically, the author would recommend the Johnson reference.' Of importance in this paper's discussion is the ability to alter the distribution of contact forces and thus, shear forces. This is accomplished through a movable pivot or point of rotation, thus dividing forces into proximal and distal. The position of the pivot either increases proximal forces, thus minimizing anterior translation of the tibia or allowing a more typical distal ' Chairman. Physical Therapy Department, Indiana Central University, 1400 E. Hanna Avenue, Indianapolis. IN 46227. placement, thus allowing a degree of anterior translation. The antishear device is shown in Figure 2. The easiest explanation of how this device works is to recognize that the more proximal placement of the pivot point the greater the contact on the proximal pad. Thus, w$ would typically use more proximal placements to minimize anterior translation. It should be remembered that contact increases tremendously with these proximal placements and we must monitor the patient's reactions and output carefully. This is particularly true of postoperative patients in which the tibia1 tubercle may be a sensitive area. One of the difficulties with the use of this apparatus is that its effectiveness is most frequently related to the perception of the patient. Most patients will indicate that the device feels very comfortable and makes them feel very secure during exercise. They may respond that they no longer are apprehensive during the extension and that "Boy, this feels good!" One objective means of quantifying the effectiveness of the device is to examine the correlation between the shape of the quadriceps torque curve with the unit in place versus the single pad. This was alluded to previously and was published in the May 1983 issue of Physical the rap^.^ Figure 3 shows an anterior cruciate curve with a single pad while Figure 4 demonstrates the same patient's Cybex tracing utilizing the antishear device. As one can see, the Figure 4 curves are approaching normalcy rather than demonstrating the active anterior drawer as seen in Figure 3.
JOSPT May 1986 CLINICAL USE OF THE JOHNSON ANTI-SHEAR DEVICE Fig. 1. Cybex Graph of an anterior cruciate deficient knee patient's 60 /sec torque curve, recorded at 25 mm chart speed. Notice the abnormality in the quadricep's curve which is indicative of anterior cruciate and capsular involvement. The dip which occurs in the area of peak torque after the initial spike is the "active anterior drawer. "
306 MALONE JOSPT Vol. 7, No. 6 the pads. The therapist then tightens all of the knobs prior to exercise. Seven recommendations are provided for the use of this device: Recommendation One Initial pivot position is recommended to be midway, thus equally dividing proximal and distal contact forces. This may be all that is necessary for controlling most anterior cruciate patients. Recommendation Two Fig. 2. The antishear device has three controlling knobs. Knob 1 allows the therapist to manipulate the distance between pads. Knob 2 allows the device to be properly placed on the patient. Knob 3 controls the placement of the pivot, thus controlling the amount of contact force allowed to be dispersed through each pad. The placement of the pivot allows the antishear device to approximate the single pad (Fig. 5). As the pivot is moved proximally into a split position, the patient now experiences an equalization of contact forces (Fig. 6). A very proximal placement of the pivot concentrates the contact proximally (Fig. 7) as one would desire in treating anterior cruciate deficient knee patients. HOW TO USE THE DEVICE An examination of Figure 2 shows three knobs which allow the therapist to fit the device to the patient. The first step in the process is to loosen the aluminum locking knob which allows the therapist to adjust the distance between pads. The second knob (to the input adapter) allows the therapist to position the pads and tighten the straps, thus securing the device to the patient. The third knob allows the therapist to adjust the pivot position or distance between the pivot and The patient should work submaximally until he or she is very comfortable with the device. Most patients feel very secure and often say that this feels better than any pad or unit they have used previously. Recommendation Three Cover the upper pad with temper foam (Kees- Goebel Medical, 4954 Provident Drive, Cincinnati, OH 45246) (T-41 density). This foam will not "bottom out" and thus allows a much better fit between the tibia and the pad. This makes the exercise much more comfortable and provides a much better fit from patient to patient. It would have been very nice if the upper pad had been able to rotate, thus allowing a fit from patient to patient, but this would have provided a weak point in the device and would have presented many maintenance problems (Fig. 8). Recommendation Four With some patients, work only the quadriceps when using the double pad. This is particularly true when dealing with patients who are very lax and have severe secondary capsular involvement. When using the device, we control anterior shear very well but must recognize that when we initiate hamstring contraction the device has a tendency of placingan anterior shifting force on the proximal pad through hamstring contraction. Thus, you may wish to only exercise the quadriceps using the typical double pad placement and use a single pad for hamstring exercise. R~~~~~~~~~~~~~ Five Do not compare torque values generated from a single pad to that of the antishear or double pad device. We typically see slightly decreased torque
Fig. 3. Cybex tracings of an anterior cruciate deficient knee patient at 180 /sec and 25 mm chart speed, using the typical single pad with a distal placement. + Fig. 4. Curves showing Cybex tracings of the same patient in Figure 3 but using 180 /sec and a 25-mm chart speed, but using the double pad or antishear device with a split placement (pivot equidistant from the pads).
308 MALONE JOSPT Vol. 7, No. 6 Fig. 5. Placement duplicating the single-pad placement of the typical Cybex. production, but may in fact see increased production with those patients who have been unable to exert safely with a single pad. Recommendation Six Be very careful when using very proximal pivot placements. Great proximal contact may be very painful, particularly following intraarticular surgeries involving the quadriceps insertional area. Recommendation Seven It is important to fit the use of the antishear device to the individual patient. The split position (Recommendation One) may be the best for most uses but you must fit the needs of the patient. You may use this device in suspected cruciate damaged knees, following ligamentous reconstructions, ligamentous repairs, or in suspected ligamentous instabilities. Fig. 6. Pivot placement (split position) #fat gives equal contact between the proximal and distal pads, thus decreasing the amount of anterior shear stressing the ligamentous structures. CONCLUSION The Johnson Anti-Shear Device does an excellent job of preventing the anterior translation of the tibia on the femurduring quadriceps exercise. It may not provide a proper format for hamstring exercise in those patients who have a Very lax anterior cruciate deficient knee; hence, in those patients it is important to exercise the hamstrings using a single pad rather than the antishear device, or simply lower the pad placement illustrated in Figure 5. The antishear device allows the therapist to control more effectively the tibia during extension movements and thus more safely develop quadricep strength in patients with ligamentous laxities. Subjectively, patients feel secure and express satisfaction at being able to exercise without pain. Objective data may be provided through the examination of the quadriceps curve as generated during isokinetic contractions on a Cybex
JOSPT May 1986 CLINICAL USE OF THE JOHNSON ANTI-SHEAR DEVICE 309 Fig. 7. Proximal placement of the pivot which minimizes the amount of anterior shear allowed during extension moments. II. This adds to the subjective findings expressed above. Although the device does control anterior translation, it is the author's opinion that this device should not permit too early quadriceps strengthening programs following anterior cruciate reconstructions. Rather, it should allow safer implementation within the appropriate time frames. As described in Recommendation Two, the therapist should be certain to have the patient work submaximally prior to asking for full-maximal contractions. This is particularly true when working with patients following reconstructive surgeries and is the exercise of choice (submaximal levels) in the early phases of rehabilitation following reconstruction. Hypothetically, the therapist could begin the Fig. 8. The upper pad is covered with T-41 temper foam. This allows much more comfort, particularly with proximal placements of the pivot. rehabilitation with extremely proximal pivot placement, thus minimizing the anterior translation maximally. The therapist could then progressively increase the stresses on the cruciate reconstruction by slowly moving the pivot point distally during the rehabilitation program. It is the author's recommendation not to proceed beyond a split of midposition with these patients. REFERENCES 1. Johnson D: Controlling anterior shear during isokinetic knee extension exercise. J Orthop Sports Phys Ther 4:23-31,1982 2. Malone T, Mangine R: Clinical correlations of the quadriceps torque curve and the anterior cruciate (abstract) Phys Ther 63:762, 1983