Hip Continuous Passive Motion Device (The Ohio State University)

Hip CPM Team with Device in Photo

Design Team for Hip Continuous Passive Motion Device

Nick Black, Meagan Ita, Beth Schlegel, Andy Turner, Michael Vignos


Patients who have surgery to repair the hip acetabular labrum require post-op continuous passive motion therapy to reduce pain, joint adhesions, and muscle tightness. This therapy is performed by physical therapists (PTs) and caregivers for 12 minutes, twice a day for 2-3 weeks post-op. Over 220 of these procedures are done every year at the Wexner Medical Center. To reduce the physical strain and time constraint placed on PTs and caregivers we designed a mechanical device that replicates this therapy. This device utilizes adjustable counterweights to offset the weight of the patient’s leg and to provide a stable circular motion. Currently we have conducted several testing procedures on healthy individuals. The overall results of testing indicate overall satisfaction scores of 29-34 out of 40, indicating reasonable approval of the device. However, results also suggest that future robustness and aesthetic modifications are needed prior to wide-spread marketing of this device.


The hip acetabular labrum is a triangular fibrocartilaginous structure that outlines the bony rim of the acetabulum to prevent femoral translation and to reduce contact pressure within the joint (Figure 1) (1,2,3). Damage can occur to this tissue, typically in highly active populations, through repetitive, high-impact joint loading (1,4). This damage causes pain and functional limitations such as difficulty walking or climbing stairs (1). Surgery is typically required to repair the damage (3). These surgeries are fairly common with 220-240 procedures performed by a single hospital (The Wexner Medical Center at Ohio State University) and the number of labral repair surgeries diagnosed each year is constantly increasing (5).


Image of hip acetabular labrum

Figure 1: Hip acetabular labrum surrounding acetabulum.

Immediately following the procedure to repair the hip labrum, patients begin a physical therapy regiment that includes a hip continuous passive motion (hip CPM) therapy. This post-operative procedure helps reduce the formation of scar tissue and therefore increases mobility, maintains range of motion, and decreases inflammation and pain. The hip CPM therapy consists of leg rotation for 3 minutes clockwise and 3 minutes counterclockwise at both 30 and 70 degrees of hip flexion (Figure 2). This procedure is typically performed twice a day by either a PT or the patient’s caregiver, depending on the location the therapy is being performed (3).

Demonstration of Hip CPM Therapy

Figure 2: Hip CPM therapy performed at 30 degrees (left) and 70 degrees (right) hip flexion

As the prevalence of labral hip tears increases, PTs and caregivers are forced to spend more time performing this procedure. This therapy is also very strenuous for these individuals to perform since they must support the leg weight during the entire procedure. An assistive device to perform this therapy would help meet the needs of affected patients for a fast and effective recovery and remove the burden of performing this procedure from PTs and caregivers.


The goal of this project is to create an automated device that accomplishes the same motion carried out by a PT, while maintaining comfort for patients.


For our final design we attempted to utilize a simplistic design while maintaining essential  device functionality (Figure 3). The primary components of the final device are described in detail below.

Actual device and model of device

Figure 3: Completed hip CPM device prototype (left) and model of the final hip CPM design (right)

Device Frame

The device frame was constructed from 1.5 in. x 1.5 in. square hollow 6063-T6 aluminum tubes (Figure 4). Hollow aluminum was chosen to minimize the weight of the frame to allow for ease of transportability. The height of the frame, 16.5 in., was chosen to ensure comfort for a wide range of patients.  Device height was chosen so that all patients between the 5th percentile female and the 95th percentile male would have a hip flexion angle between 30 degrees and 70 degrees (7). In future designs we plan to incorporate adjustable device height to accommodate a varying hip flexion angles for patients.

Device Frame

Figure 4: Model of device frame


The use of a counterweight is essential to the functionality of our device to create a smooth, continuous rotation. It offsets the weight of the leg at an equal distance from the center of rotation. This allows the motor to do little work to lift the leg to the top of the rotation and allows the leg to be lowered at a constant speed, rather than being accelerated by gravity.

Currently the counterweight consists of metal weightlifting plates of 5 lbs. and 2.5 lbs. (Figure 5). This design allows PTs to modify the weight quickly and easily to match the patient’s leg weight. In future designs the counterweight would consist of specialized pieces that are similar in weight to current counterweights , but more visually appealing to patients. These pieces would still be easily adjustable.

Counterweight component

Figure 5: Model of counterweight component


The motor chosen for our device is a brushless DC stepper motor (Anaheim Automation, Inc.). From dynamic torque calculations we determined our motor must produce a dynamic torque of 6.51 oz-in to accelerate the device from 0 to 40 RPM in 30 seconds, when a 40 lb leg is being rotated. Ideally, we will have 0 static torque since our counterweight will be set equal and opposite to the weight of the leg. However, there is a possibility the weight of the leg and the counterweight will be off by a maximum of 2.5 lbs. In this case the motor must produce 150 oz-in of torque. The motor chosen has a peak torque of 694.5 oz-in so it will be able to provide our desired torque.

Rotational Component

The driving shaft, at the top of the device, is driven by the motor through a belt-gear system. The motor has a gear directly attached to it. From a gear directly attached to the motor, a belt runs up to a second gear attached to the top rotation shaft to drive the rotation of the leg (Figure 6). The current radius of rotation of the device is 3.75 in. This was chosen because it is the radius the PT uses for patients immediately post-op (5). As the patient heals, the therapist increases the radius. For our initial prototype we started with a single rotation radius so we chose the most conservative option. In future designs, we plan to incorporate an adjustable radius.

Rotational component of device

Figure 6: Model of rotational component of device

Leg Sling

The leg sling was designed with carbon fiber tubes as the side supports (Figure 7). These tubes allow for no visible deflection when the patient’s leg is in the sling. The sling portion was sewn out of cloth fabric with buckles similar to those of a backpack sewn on to attach the sling to the carbon fiber support rods. The use of the backpack buckles allows the sling height to be adjusted to improve patient comfort.

Leg sling model

Figure 7: Model of leg sling

The use of this leg sling allows patients to rest their leg without being locked in position. This allows patients to freely internally and externally rotate their leg to allow for maximum comfort. This feature is enhanced by the ball bearing that is attached to the faceplate of the leg sling. This bearing allows patients’ legs to stay parallel to the surface the device is resting on and allows patients’ legs to naturally internally and externally rotate during the procedure with negligible opposing force. This ease of rotation is necessary for patient comfort.


Before we could evaluate our hip CPM device clinically, we conducted various engineering tests to ensure the safety and functionality of our design. These engineering tests fell within three main categories (measurements, stability, and functionality) governed by our original design objectives and requirements. A test in the first category analyzed the projected hip flexion angles of the 5th percentile female to the 95th percentile male and revealed flexion angles between 12.5 and 43.6°. These angles fall below the 30-70° hip flexion target and therefore may result in a therapy that is not as effective as it could otherwise be with some device dimension modification. Undershooting rather than overshooting these angles, however, protects the safety and comfort of the patient, as a too steep angle can counteract therapeutic benefits and cause pain. Device frame movement was assessed within the stability category of testing, and no frame instability problems were noted both during standstill and during rotational motion. As a test within the functionality category, we conducted tests with varying loads to evaluate the continuous smooth motion of our device by comparing times to complete the first half and second half of one full revolution (Figure 8). Our test results revealed that for revolutions evaluated with no load, 10 lbs, and 20 lbs, differences between the first half (0-180°, with 0° defined as the apex of rotation) and second half (180-360°) of rotation averaged over 12 trials did not exceed 0.14 seconds.

Image showing the smoothness test performed

Figure 8: Test performed to show the smoothness of device. This test was performed to determine if the time it took to lift the weight was equal to the time it took to lower the weight. Zero difference in these times is desired

Although we were not yet able to conduct hip CPM trials on post-op patients, we did have the opportunity to conduct partial trials on healthy individuals (Figure 9). For these individuals, we conducted a post-trial user satisfaction survey to rate perceived satisfaction with dimensions, weight, ease of adjustment, safety, durability, ease of use, comfort, and effectiveness of the device on a 1 (not satisfied at all) to 5 (very satisfied) scale. All 6 individuals scored the CPM device in a 29-34 range out of a 40 possible points. Within this breakdown, highest scores (all 4 or 5) were recorded in the categories of dimension and device effectiveness satisfaction, while the lowest scores (all 3 or 4) were recorded in the weight satisfaction category. Although this sample population is preliminary and preclinical, we were encouraged to record ratings of 4 or 5 in the category of effectiveness, as this aids us in validating the ability of our device to functionally replicate the manual hip CPM therapy. Additionally, we believe that as our prototype evolves its performance in many of these areas, as well as quantitative outcomes for the previously described engineering tests, will be drastically improved. Future work and modifications (as described in the next section), and the continuation of a robust testing plan, will allow us to test this device on current hip labral repair patients.


The testing conducted on our device revealed a high degree of perceived effectiveness and appropriate size dimensions, validating two of our primary objectives. The design and construction of the device was also kept well within the budget we were allotted. Out of a total budget of $1500, the device cost just $843.58, meeting yet another of the original objectives: cost efficacy. By finishing the prototype device $656.42 below budget, we feel that there is great potential for future modifications to improve the device, while still maintaining the original $1500 budget.

Though our device proved effective and functional, there are changes that must be made before attempting to integrate the device into a clinical setting. The first of these is welding the frame, rather than using screw and nut connections. This would increase stability, while also improving aesthetic appeal for patients. Creating a plastic covering for the motor and power supply will also ensure user safety. As previously mentioned, we would like to modify the upright frame that allows the user or therapist to adjust the height of the leg support, changing the angle of the hip. This would allow for each patient to set the device within the commonly accepted 30-70 degrees of hip flexion.

We believe that our device is a functional and effective replacement for a PT or caregiver in administering hip continuous passive motion to post operative patients. Thus far, healthy test subjects, including a practicing PT, have given high survey ratings for effectiveness, validating this belief. We look forward to the opportunity of further clinical testing, especially with post-operative patients, and the possibility of implementation in a clinical setting.


1. Groh, M.M., Herrara, J. (2009). Curr Rev Musculoskelet Med. 2,105-117.

2. Ranawat, A.S., Kelly B.T. (2005). Operative Techniques in Orthopaedics. 15,160-174.

3. Lewis, C.L., Sahrmann, S.A. (2006). Physical Therapy. 86,110-121.

4. Mason, J.B. (2001). Clin Sports Med. 4,779-90.

5. DeWitt, John (DPT, ATC, SCS). Personal Interview. 2012, Oct. 8.

6. Demers, L., Weiss-Lambrou, R., Ska, B. (2002). The Quebec User Evaluation of Satisfaction with Assistive Technology (QUEST 2.0). Technology and Disability. 14,101-105.

7. Fryar C.D., Gu Q., Ogden C.L. (2012). Vital Health Stat. 11(252).

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