Torque Measures of Common Therapies for the Treatment of Flexion Contractures

The Journal of Arthroplasty Vol. 26 No. 2 2011

Torque Measures of Common Therapies for the Treatment of Flexion Contractures Timothy L. Uhl, PhD, ATC, PT, FNATA,* and Cale A. Jacobs, PhD, ATC*y

Abstract: Efficacy of knee flexion contracture treatment protocols is dependent on the torque applied to the joint, but to date, no published reports have evaluated the torque applied by the available treatment options. The purpose of this study was to measure the torque applied by physical therapists (PTs), home exercises, and mechanical therapy devices. An instrumented test leg recorded peak torque applied by 14 PTs performing knee extension mobilization, 2 home exercises, and 3 types of mechanical therapy (dynamic splint, static progressive stretch, and patient-actuated serial stretch). Physical therapists applied 68.0 N m, patient-actuated serial stretch mechanical therapy applied 53.0 N m, and the other therapies ranged between 4.6 and 12.4 N m. The reported torque values can be used to help improve the efficacy of flexion contracture treatment protocols. Keywords: extension loss, flexion contracture, manual therapy, mechanical therapy, home exercise. © 2011 Elsevier Inc. All rights reserved.

Before total knee arthroplasty (TKA), approximately 35% of patients have been reported to present with flexion contractures 6° or greater [1]. Patients with preoperative flexion contractures demonstrated significantly greater risk of experiencing poor outcome scores and decreased walking distance after surgery [1]. By limiting a patient's ability to properly accept weight during gait, flexion contractures of even 1° may negatively affect clinical outcomes [1]. Ritter et al [2] reported that postoperative flexion contractures after TKA led to poorer outcomes related to pain, walking, stair-climbing, and function. Patients with flexion contractures often walk with a bent-knee gait, thus, increasing strain on the quadriceps and increasing contact forces in the patellofemoral joint [3]. Walking distance is reduced as the disadvantaged position and increased strain during bent-knee gait may lead to either quadriceps weakness and/or an earlier onset of quadriceps fatigue [4]. Furthermore, an inability to achieve full postoperative extension has been reported to lead to a more rapid degeneration of the contralateral knee [5]. Despite the negative ramifications on pain and function, the treatment options available for TKA patients with From the *Department of Rehabilitation Sciences, College of Health Science, University of Kentucky, Lexington, Kentucky; and yERMI Inc, Atlanta, Georgia. Submitted July 7, 2009; accepted December 8, 2009. No benefits or funds were received in support of the study The second author is an employee of ERMI Inc, Atlanta, Ga. Reprint requests: Cale A. Jacobs, PhD, 441 Armour Pl NE, Atlanta, GA 30324. © 2011 Elsevier Inc. All rights reserved. 0883-5403/2602-0025$36.00/0 doi:10.1016/j.arth.2009.12.007

flexion contractures are limited. Revision of a stiff TKA is not recommended within the first 6 months after the primary procedure and often does not occur until well after the first year [6-8]. During the first postoperative year, conservative treatments including physical therapy, home exercise programs, and home mechanical therapy are often used to treat and minimize the occurrence of flexion contractures. For patients who have failed standard conservative treatment for 2 or more months, focused treatment protocols including physical therapy and the use of custom knee devices has been demonstrated to effectively treat flexion contractures [9]. The effectiveness of a given treatment to reduce flexion contractures is a function of the applied torque, as well as the duration and frequency of the treatment [10]. Mechanical therapy device manufacturers provide treatment protocols based on duration and frequency that only provides 2 of the 3 factors necessary to properly dose a treatment intervention. Little is known about the specific torque being applied to the knee by the various therapeutic interventions often prescribed to treat knee flexion contractures. Therefore, the purpose of this study was to measure the torque applied by physical therapists (PTs) during manual therapy, home therapy exercises, and 3 types of mechanical therapy devices. We hypothesized that home exercises and mechanical therapy devices would demonstrate significantly lower torque than the torque applied by the PTs.

Methods We constructed a lower extremity model (Sawbones Soft Tissue Leg 1515-7, Pacific Research Laboratories,

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Torque Measures in Treatment of Flexion Contractures  Uhl and Jacobs

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Torque Measurement

Fig. 1. A PT performing a knee extension mobilization on the instrumented test leg.

Inc, Vashon, Wash), which consisted of a complete synthetic left lower extremity skeleton that was encased in closed cell foam representing soft tissue with a synthetic skin exterior. The femur and tibia were removed and replaced with two 1” × 2” pieces of aircraft aluminum extrusion (80/20 Inc, Columbia City, Ind). At the knee joint, the 2 pieces of extrusion representing the femur and tibia were rigidly fixed to a torque sensor (DI3N-IP500; Imada, Inc, Northbrook, Ill).The torque sensor is rated by the manufacturer to be accurate within ±0.5% from 3 to 500 N m. The knee joint was adjustable and could be locked at either 0° of knee extension or 90° of knee flexion. In an attempt to create a more realistic feel for the PTs during application of manual therapy, 2° of play was built into the system to allow for a small amount of joint motion as each test repetition was initiated. An RS232 cable ran from the sensor through the extrusion femur to the torque measurement display unit, which was then connected to a dedicated laptop with a USB cable. The torque measurement display unit was mounted to an aluminum frame located proximal to the pelvis of the instrumented lower extremity model. A wooden frame encompassed the aluminum frame to protect the table from damage (Fig. 1).

Subjects Fourteen licensed PTs participated in the study (7 women, 7 men; age = 44.3 ± 10.2 years; height = 172.9 ± 13.2 cm; weight = 72.6 ± 13.0 kg). A waiver of informed consent was approved by the university institutional review board. Eleven of the PTs were employed in a clinical outpatient setting at the time of testing, with 3 PTs primarily employed in an academic setting but had previous clinical experience. Mean years of experience was 18.2 years and ranged from 3 to 34 years.

Manual Passive Extension Mobilization The test leg was securely clamped to a plinth in a supine position with the heel cradled in a 10.16 cm (4”) bolster (Fig. 1). Physical therapists were asked to perform the mobilizations exactly as they would in the clinical setting. Twelve of the PTs used a static mobilization technique described by Kaltenborn [11], which involved a steady, consistent force application. Two PTs used an oscillatory mobilization technique described by Maitland as a grade 4 mobilization at the end range of motion [12]. In addition, they were allowed to adjust the height of the plinth and hand placement was not controlled. Twelve of the PTs placed one hand on the anterior aspect of the distal femur and the other hand on the anterior aspect of the proximal tibia, whereas 2 PTs placed both hands on the anterior aspect of the distal femur. Physical therapists were instructed to perform five 10-second mobilizations, and the peak torque of each repetition was corrected for the amount of torque created by gravity, and the peak torque generated by the therapist was recorded for statistical analysis. The PTs were instructed to gradually increase the force during the first 2 seconds of each repetition in a manner similar to what they would perform clinically. The mean peak torque of the middle 3 repetitions for each PT was then calculated and used for analysis. The coefficient of variation was calculated for the 3 test repetitions for each PT to assess the consistency at which torque was applied. Home Exercises A single PT performed torque testing of 2 commonly used home exercises: heel prop and prone hang exercises. For the heel prop exercise, the test leg was securely clamped to the plinth in a supine position. The torque sensor was reset to zero, and the test leg was placed in the test position. A standard pillow was placed between the ankle of the test leg and the plinth, and peak torque was recorded with no additional overpressure applied. The pillow was then removed, and the testing procedure was repeated 2 additional times (Fig. 2). During the prone hang exercise, the test leg was securely clamped to the plinth in a prone position with the knee joint, low leg, and foot in an unsupported position. As with the heel prop exercise, 3 repetitions were performed with peak torque being zeroed before placing in test position and the peak torque was recorded. Additional external resistance is often used clinically to increase the amount of torque applied during the prone hang exercise, and these commonly used modifications were also assessed. Additional trials were performed using the same methods but with the addition of a low top, size 12 court shoe (Nike, Inc, Beaverton, Ore) and the same shoe with an additional 0.91-kg (2-lb) ankle weight.

330 The Journal of Arthroplasty Vol. 26 No. 2 February 2011

Fig. 2. The test leg positioned for the heel prop exercise.

Mechanical Therapy The same PT that performed the testing of the home exercises also performed the testing of 3 types of mechanical therapy devices. The 3 types of mechanical therapy that were tested included a dynamic splint (Advance Dynamic ROM Knee; Empi, St Paul, Minn) (Fig. 3), static progressive stretch (SPS) brace (Static-Pro Knee; DeRoyal Industries, Powell, Tenn) (Fig. 4A), and a patient-actuated serial stretch (PASS) device (ERMI Knee Extensionater; ERMI, Inc, Atlanta, Ga) (Fig. 5). The test leg was securely clamped to the plinth in a supine position, and the devices were applied to the test leg per the manufacturer's instructions. After each device was applied, the amount of external torque was maximized for that specific device. Peak torque was corrected for the amount of torque created by gravity and recorded.

Fig. 4. (A) A static progressive stretch brace (Static-Pro Knee; DeRoyal Industries) applied to the test leg. (B) The final brace position after adjusting to the maximum torque settings.

Mean peak torque, coefficient of variation, and range of mean peak torque values were calculated. Mean peak

torque for the 8 therapies were compared using a 1-way analysis of variance. All calculations were performed using SPSS version 17.0 (SPSS Inc, Chicago, Ill), and the α level was set a priori at P b .05.

Fig. 3. A dynamic splint (Advance Dynamic ROM Knee; Empi) applied to the test leg.

Fig. 5. A PASS device (ERMI Knee Extensionater; ERMI Inc) applied to the test leg.

Statistical Analysis

Torque Measures in Treatment of Flexion Contractures  Uhl and Jacobs

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Fig. 6. The applied torque significantly differed (P = .002) between the heel prop and prone hang exercises with and without the addition of a shoe and/or 0.91 kg weight, dynamic splint, SPS device, PASS device, and PTs.

Results The 8 therapies were significantly different (P = .002), but pairwise comparisons could not be made due to a lack of intertrial variability with the mechanical and home therapies. Mean peak torque applied by PTs was 68.0 ± 17.5 N m, and the PTs were very consistent in their torque application as evidenced by the low coefficient of variation (mean, 5.6%; range, 1.5%-13.5%). Peak torque of the home exercises were as follows: heel prop = 4.6 N m, barefoot prone hang = 6.2 N m, prone hang with shoe = 10.1 N m, and prone hang with shoe and weight = 12.4 N m. Peak torque applied by the mechanical therapy devices were dynamic splint = 7.2 N m, SPS brace = 10.4 N m, and PASS device = 53.0 N m (Fig. 6).

Discussion The purpose of this study was to measure the torque applied by PTs during manual therapy, home therapy exercises, and 3 types of mechanical therapy devices. We hypothesized that home exercises and mechanical therapy devices would demonstrate significantly lower torque than the torque applied by the PTs. The results of this study partially support our original hypotheses. It has been previously reported that the median torque necessary to maintain terminal knee extension in patients with flexion contractures is 9.0 N m [13]. The results of the current investigation suggest that joint mobilizations performed by PTs, a PASS mechanical therapy device, prone hang exercises performed with a weight or shoe, and an SPS device produce enough

torque to hold the knee in full extension, whereas the other therapies do not. Physical therapists often use high-grade mobilizations to help improve range of motion. High-grade mobilizations involve greater force application, which is generally applied for approximately 30 seconds, with 2 to 6 bouts per session [14]. Although high-grade mobilizations may result in immediate improvements in the patient's range of motion, these gains have been reported to be only temporary [10,15,16]. Several groups of authors have suggested that tissue lengthening resulting from short-duration mobilizations is merely a function of the viscoelastic property of the connective tissue and that the tissue will return to its shortened state after the force is removed [10,15,16]. If one considers the “torque dosage” or the total amount of torque applied to the joint over the course of a week, then it becomes clearer how adjunctive home therapy is required for permanent elongation of the connective tissue and lasting gains in range of motion. The effectiveness of a given treatment to reduce flexion contractures is a function of the applied torque, as well as the duration and frequency of the treatment [10]. Based on the average torque of 68 N m applied for four 30-second bouts performed 3 times a week, a resulting 408 N m min/wk would be applied by a PT when treating a knee flexion contracture. Assuming that postoperative patients would be able to routinely tolerate 100% of the torque applied during either the heel prop or prone hang exercises and 50% of the peak torque measured with the mechanical therapy devices in

332 The Journal of Arthroplasty Vol. 26 No. 2 February 2011 this investigation, the sustained duration of force application combined with daily use results in a greater total torque dosage for the knee when compared to the dosage during physical therapy alone (Table 1). The reduced torque dosage associated with joint mobilizations alone may potentially explain why other authors have concluded that short physical therapy sessions may not be enough to sustain improvements in joint range of motion [10,15-17]. Although the torque dosage appears to vary between treatments based on the applied torque and frequency and duration of treatment, the optimum torque dosage required for lasting tissue elongation has yet to be determined. Previous literature addressing the ability of adjunctive mechanical therapy to treat flexion contractures is limited to a few studies with limited sample sizes. Steffen and Mollinger [13] reported the use of dynamic splinting in addition to passive range of motion exercises did not significantly improve the range of knee extension in a group of 18 elderly nursing home patients. In that study, patients used a dynamic splint (Knee Extension Dynasplint, Dynasplint Systems, Inc, Severna Park, Md) with a consistent torque setting of 6.1 N m for 3 h/d, 5 d/wk. In addition, PTs performed extension mobilizations for approximately 6 min/d, twice per week [13]. The resulting torque dosage for these patients was 6306 N m min/wk and was ineffective at improving knee flexion contractures. Other case series and case reports evaluating mechanical therapy devices have reported successful treatment. In a series of 4 patients, Hepburn [18] reported that patients treated with a dynamic splint (Dynasplint) for 8 to 12 h/d, 7 d/wk reduced flexion contractures by 49%. With a reported torque setting of 8 N m, the resulting torque dosage for this small series of patients would then range from 26 880 to 40 320 N m min/wk. In a recent case report, Finger and Willis [19] reported successful Table 1. Calculated Torque Dosage for Each of the Knee Flexion Contracture Treatments Torque Time Bouts Days Dose of torque (N m) (min) Per Day Per Week (N m min/wk) PASS device 26.5 Prone hang— 12.4 shoe & weight Prone hang— 8.4 shoe SPS brace 5.2 Dynamic splint 3.6 Prone hang— 6.2 barefoot Heel prop 4.6 PT extension 68.0 mobilization

10 10

6 3-6

7 7

11 130 2604-5208

10

3-6

7

1764-3528

30 120 10

3 1 3-6

7 7 7

3276 3024 1302-2604

3-6 3-5

7 2-3

966-1932 204-510

10 0.5

For the mechanical therapy devices, 50% of the peak torque measured in the current study was used to calculate the dose of torque, as it was assumed that patients would not be able to tolerate the true maximum torque generated by each device.

resolution of a 12° flexion contracture with the use of a dynamic splint (Dynasplint) set to 8 N m of torque for 6 to 8 h/d in addition to joint mobilizations performed during 28 physical therapy visits for a 12-week period. The resulting torque dosage ranged from 20 556 to 27 276 N m min/wk and was similar to the lower end of the range used by Hepburn. The large variability in torque dose between these studies may potentially be involved with why mixed results have been demonstrated when treating flexion contractures with dynamic splints [20]. In a study of TKA patients, Mont et al [21] reported that patients with flexion contractures were successfully treated with a custom device used in 30-minute to 45minute intervals, 3 times per day. Unfortunately, the torque applied to the joint with this device was not reported, and the torque dosage cannot be calculated. In a series of 43 patients with postoperative flexion contractures of 10.4° ± 5.4°, Dempsey et al [22] recently reported that the degree of extension significantly improved to 2.2° ± 3.2° when patients were treated with the PASS device evaluated in the current study. Patients were instructed to use the PASS device in six 10-minute bouts of end-range stretching per day. Assuming that patients were able to use the device at approximately 50% of the peak torque values generated in the current study, the resulting torque dosage was 11 130 N m min/wk. The aforementioned results of treatment with mechanical therapy devices demonstrate the wide range of torques that have been used but do begin to point toward a potential target torque dosage. As previously mentioned, torque doses of less than 6500 N m min/wk were unsuccessful, and studies that used torque doses greater than 11 000 N m/wk demonstrated improvements in the range of knee extension. By providing a better understanding of how to appropriately adjust the intensity, frequency, and duration of each treatment, as well as how combining treatments may influence the torque dose, the torques and doses discussed in the current article may be used to help clinicians develop individualized treatment protocols for patients with postoperative flexion contractures. Future studies are certainly warranted to determine the optimal torque dosage, and although we have discussed these concepts in detail, we caution readers that the current discussion has been based on the results of laboratory data and the clinical results of very small series of patients. These laboratory results are no substitute for well-designed clinical studies. Also, future studies will not only have to determine if there is a torque dosage threshold for successful conservative treatment of flexion contractures but also whether there is a minimum level of torque necessary, regardless of frequency and duration, to move the joint to full extension. As previously mentioned, Steffen and Mollinger [13] reported that a torque of 9.0 N m was necessary to maintain full extension in 18

Torque Measures in Treatment of Flexion Contractures  Uhl and Jacobs

patients with flexion contractures, and it remains unclear if this threshold would be influenced by the severity of the contracture or by other patient factors such as the weight of the limb. This study was not without limitation. Physical therapists were asked to perform the mobilizations in the same way that they normally would when treating a patient; however, patient feedback that PTs generally use when determining the appropriate amount of force to be applied was obviously not provided by our testing system. The torque measured with the dynamic splint used in the current study did not greatly differ from those reported in clinical studies using a Dynasplint dynamic splint. We measured a peak torque of 7.2 N m with the Empi device, whereas Steffen and Mollinger [13] stated that the Dynasplint applied 6.1 N m, and Hepburn [18] and Finger and Willis [19] stated that 8 N m were applied with the Dynasplint. Home exercise torque values were derived from the mechanical leg used in this study. These values would vary based on a patient's individual height and weight. However, the results of the heel prop exercise were not unrealistic. Assuming a 200-lb (90.7-kg) male with a height of 5 ft 9in (1.76 m), we estimated a knee extension torque of 7.5 N m using 3-point bending equation, which is only 3 N m different from the torque measured in the current study. In addition, we compared the peak torque generated by each of the treatments not the mean torque seen by the knee over the entire duration of a treatment. Although it can be assumed that the amount of torque may change throughout a given treatment, this relative percent change in torque was not calculated. Most notably, the 2 different mobilization techniques used by the PTs may differ in the mean torque applied over the entire trial as one technique involves oscillating at the end range of motion compared to the other technique that involves a consistent application of force. Future studies are necessary to determine if the different application techniques influence the clinical efficacy of these techniques. Finally, in the interest of consistency of measurement, we only evaluated the maximum amount of torque that could be applied with each device. These maximal settings may be beyond the settings normally used during treatment. For example, although the thigh and low leg cuffs of the SPS brace were securely tightened, as the brace was adjusted to its maximum torque settings, the location of the brace's thigh and low leg cuffs both migrated toward the knee (Fig. 4B). The change of position undoubtedly resulted in decreased torque applied to the knee itself, but because brace migration and soft tissue compression are demonstrated clinical phenomena, we opted to include the data from that test. In summary, home exercises and mechanical therapy may be used as daily adjuncts to physical therapy to help improve joint range of motion. The results of the current

333

study demonstrated that home exercises, a dynamic splint, and an SPS brace demonstrated lower peak torque than the torque applied by PTs or a PASS device. The results of this study may be used to help individualize the torque dosage for each patient. The most appropriate treatment for a patient will be dictated by the amount of time the patient can dedicate per day to improving motion, how well the patient tolerates the specific exercise/device, and whether the patient is compliant with the prescribed protocol. Lasting gains in motion can be achieved with any of the treatments evaluated in the current study, but surgeons and rehabilitation specialists need to be cognizant of not only the duration and frequency of treatment but also torques generated by various exercises necessary for the treatment of knee flexion contractures.

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