Specific Aims



SBIR Proposal

Rotational Stability Measurement Device

For Analysis of Anterior Cruciate Ligament Reconstruction

K. Wagner

S. Bechtold

K. Dillon

UPMC Department of Orthopedic Surgery

Aircast, Inc.

Table of Contents

A. Specific Aims 3

B. Background/Significance 4

Importance 4

Economic and Market Factors 6

Facilities used 6

C. Investigators/Relevant Experience 7

D. Experimental Designs and Methods 8

Device Development 8

Human Factors Considerations 11

Data Collection and Processing 12

Validation and Accuracy 13

Preliminary Studies 13

Impact and Significance of Design 15

E. References 15

F. Gantt Chart 17

G. Appendix 1 19

A .Specific Aims

The goal of this project is to develop and test a novel device to determine the rotational stability of the knee after an anterior cruciate ligament (ACL) reconstruction.

A.1 Design a measurement device to accurately, repeatably, and cost-effectively measure axial rotation of the knee

A design will be obtained that measures the rotational stability of the knee accurately and safely. In doing so, a quantitative measurement can be determined to evaluate single bundle reconstruction of the knee versus double bundle reconstruction of the knee. This will determine the best procedure for full recovery.

A.2 Fabrication of a prototype

The design of the rotational stability measurement device prototype will optimize cost of materials, cost to consumers, weight, safety, and reliability. This will result in a product that can be easily distributed for widespread use in the clinical community.

A.2.1 Lower Leg device

A device will be fabricated that rigidly fixes the ankle. This device will also allow for the monitoring of an applied moment during testing and the recording of rotational measurements.

A.2.2 Upper Leg brace

Since axial rotation of the knee results in coupled motion of the hip, a device will be fabricated to restrict the hip joint. This device will be able to fix the hip and knee joints at various flexion angles and also adjust for patient size.

A.3 Obtain and validate measured rotation

After designing and fabricating the device, the first phase of testing for the rotational stability measurement device prototype will include normal subjects with no history of ACL injury. This will yield a baseline of normal knee rotation in healthy knees. This data will be compared against those of a second phase of testing, which will determine the rotational stability of a double bundle vs. single bundle reconstructed knee.

B. Background/Significance

B.1 Importance

The anterior cruciate ligament (ACL) connects the femur to the tibia and functions to limit the degree of extension of the knee and the internal rotation of the tibia. The ACL consists of two bundles: the anteromedial bundle (AM) and the posterolateral bundle (PL). The AM bundle is said to provide translational stability to the knee while the PL bundle is said to provide rotational stability to the knee.

ACL injuries occur in 1 in every 3000 Americans per year, most of which happen during sports activities. Since an injured ACL interferes with daily life by causing instability and pain at the knee, surgery is often required to reconstruct the anatomy of the ACL and restore normal knee function.

Current reconstruction methods involve either a single or a double bundle technique. Single bundle ACL reconstructions have been performed for many years. This method restores only the AM bundle and, therefore, only restores the translational stability of the knee. The single bundle approach has been shown to lead to approximately 20-25% of patients being unsatisfied with the reconstruction results. Reinjury to the reconstructed knee is commonly caused by the rotation of the knee such as during a pivot turn in basketball. Because of this a second reconstruction method has been developed in recent years. The double bundle reconstructive method restores both the AM and PL bundles, making the reconstruction more anatomically correct.

Devices currently exist to measure the success of an ACL reconstruction. However they only measure the translational stability of the knee. For example, the KT1000 and the Aircast Rolimeter work in similar ways to measure anterior posterior translation of the reconstructed knee.

Since the force distribution between the AM and PL bundles varies with the degree of knee flexion, a single bundle reconstruction of the ACL does not restore complete stability to the knee. Therefore, a double bundle approach to reconstruction, which reconstructs the anatomy of both the AM and PL bundles, may be superior to the single bundle approach. However, since the double bundle approach is more complex than the single bundle approach, it is not as common. In order for the double bundle reconstruction approach to become the standard approach, more evidence must exist that the double bundle approach is superior to the single bundle approach.

The recent development of this novel double bundle reconstruction technique has necessitated the creation of a device to measure the rotational stability that is supposedly restored by the PL bundle. There is currently no quantitative analysis available to measure the rotational stability of the knee. Clinicians currently use a subjective approach to measure any rotational laxity. The standard knee evaluation method is the IKDC protocol, which incorporates the patients’ rating of their symptoms and activity level with a clinician’s evaluation of the knee. These measures help determine if a reconstruction is needed or if a reconstruction was successful.

B.2 Economic and Market Factors

Statistically, there are a large number of ACL reconstructions done each year. Updating the current procedure requires changes in standards, measurement collection, and methods. An improved method that increases success rates will be adopted by the majority of orthopedic surgeons. This will lead to an increasing demand for the rotational stability device to measure the new parameters incorporated with latest procedure.

Two current measurement devices for anterior drawer of the knee are the KT-1000 (MedMetric Corp.) and the Rolimeter (Aircast Corp.). These devices range in cost from $850 to $3800 and are limited in that they are only able to measure anterior posterior translation. Our rotational stability device will range from $4,000 to $5,000 creating a market of about $200 million to $250 million. In the future, to capitalize on the need for both measurements, the device will allow for both translation and rotation measurements, thus eliminating the need for the clinic to purchase two devices.

The cost of the rotational stability measurement device will be competitive in today’s medical device market. The device will help to eliminate the extra cost associated with reinjury of an improperly reconstructed ACL by identifying the most effective reconstruction method.

B.3 Facilities used

The Ferguson Lab in the Department of Orthopedic Surgery will be used for preliminary testing. The facilities at Aircast will be utilized for development of the boot. The machine shop located in Benedum Hall will also be consulted for design development and fabrication.

C. Investigators/Relevant Experience

Kara Wagner has academic knowledge of the physiology of the human body and the mechanics of the body’s joints through the courses Human Physiology and BioMechanics I, II, and III taken at the University of Pittsburgh. She has an understanding in the study of body injury, rehabilitation, and safety from the courses of Human Factors and Ergonomics. In addition, she has skill with the software Matlab, LabVIEW, Solidworks, and FloWorks. As a former employee of Aircast Incorporated, she is knowledgeable on the background as well at the structure/function of the Pneumatic Walker used in the prototypes. Miss Wagner will obtain a Bachelor of Science in Bioengineering concentrating in Biomechanics and Orthopedics from the University of Pittsburgh. Her role in this project was to investigate background material and interface with the machine shop on the fabrication of the leg brace.

Katie Dillon has academic knowledge in the areas of physiology and biomechanics from courses including Human Physiology, Mechanical Principles of Biological Systems, the Biodynamics of Human Movement, and the Biomechanics of Organs, Tissues, and Cells. She completed her summer REU at the Musculoskeletal Research Center, where she gained practical research experience in biomechanics, specifically the biomechanics of finger coordination. She is currently working in the Medical Virtual Reality Center where she studies balance disorders. Her role in this project was to execute and evaluate the preliminary testing done with the device.

Stephanie Bechtold has academic knowledge in physiology and biomechanics from courses including Human Physiology and Biomechanics of Human Movement. Her research experience has been in the kinematics of the knee joint, as measured using a robotic testing system. She is currently employed by the Musculoskeletal Research Center, in the Department of Bioengineering at the University of Pittsburgh, and recently won Third place in the B.S. Student Paper Competition at the ASME-IMECE congress in Anaheim, CA. Her role in this project was to design and develop a SolidWorks model of the leg rest.

The principal investigators for the design project are the Ferguson Lab of the University of Pittsburgh Medical Center. The project is headed by Dr. Volker Musahl and guided by Dr. Thore Zantop. Co-investigator of the project is Aircast Corp. who will aid in financial support of the project.

D. Experimental Designs and Methods

D.1. Device Development

The prototype of this device will be constructed using a Pneumatic Walker (Aircast, NJ). This type of brace has been proven to keep the ankle rigid, and is used mostly in fracture cases. The brace is made of durable plastic and secured by hook and loop fasteners distal to the knee joint. This allows for maximum stability of the ankle joint during testing. The brace also features inflatable aircells that maintain constant pressure around the joint, for further stability and elimination of extraneous motion of the foot within the brace. Socks or other appropriate garments will be worn during each trial to protect the patient and allow for breathability of the limb during testing. In addition, the exterior of the brace should be sterilized using ethanol or standard hospital cleaners, and aerosol cleaners should be used for the interior, soft portion of the brace. The brace is manufactured for use with either foot. It measures 12” to 20” high, depending on the size of boot used (Pediatric to Extra Large) [1]. For this project a Medium size boot will be used. Other sizes should also be considered in future studies to optimize the design for the majority of the population.

The JR3 Universal Force/Moment Sensor (UFS) will be mounted at the heel of the boot, and rigidly fastened using screws. The UFS measures forces and moments in 6 Degrees-of-Freedom (DOF). The UFS will need to be positioned so that rotation about its Z-axis results in pure axial rotation of the tibia. In this way forces that are needed to create rotation of the tibia can be accurately recorded.

The Nest of Birds magnetic tracking system (Ascension Tech.) tracks the location of its sensors in 3D space. These sensors will be placed on the distal femur, proximal tibia, and the front of the boot in order to calculate the rotation of the tibia with respect to the femur. The device bases its measurement on magnetic signals, thus metals in the vicinity should be minimized when using this device.

The forces will be applied to the heel through the UFS using a torque bar, causing a rotation of the tibia. The bar will be manufactured out of stainless steel, for its easy sterilization and rigid properties. The data recorded using the UFS and Nest of Birds are incorporated in a Matlab program to monitor the applied moment and record rotation.

The most important design consideration is the placement of the patient during the test. The patient will be fixed in a position which will comfortably eliminate coupled joint motion. The knee will also need to be fixed at various flexion angles. This necessitates the development of a secondary device to immobilize the hip joint. The patient will be placed in a supine position and the device placed under the hip, raising the femur at an angle and allowing the tibia to be supported horizontally. The device will also be adjustable to allow for various femur lengths.

The device will be fabricated of sheet acrylic to maximize patient comfort and durability, and to minimize metal components which may interfere with the data collection methods. Device components include a base, adjustable top plate, angle fixing hinge, and a standard upper leg brace (Figure 1).

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Figure 1: SolidWorks model of final design (minus upper leg brace)

The features of this design include

• Adjustable top plate that slides to accommodate different femur lengths

• Large knobs to facilitate adjustment of top plate

• Adjustable upper leg brace attached to top plate for complete immobilization of femur

• Simple hinge to adjust knee flexion angle (0, 30, 45, 75, 90°)

• Wide base with gripping surface for stability when used on examining table

D.1.1 Human Factors Considerations

The device will be used in a clinical exam room or operating room by orthopedic surgeons who perform ACL reconstructions. Therefore, the user population will be very knowledgeable of the knee’s anatomy and of the various ACL reconstructive techniques. However, the user may not be well versed in the use of the technical apparatus. The surgeons will be required to undergo training in the proper use of the device, including the proper fitting of the boot on the patient, the use of the pump to inflate the boot’s aircells, and the proper operation of the CPU program (Matlab) such that continuity is established between institutions. The device should not be used by those not trained for its use or by the patients themselves. Misuse could lead to reinjury of the patient.

In previous devices that measure anterior stability, the major hazard identified was applying too much force to the knee joint. Therefore, a 10Nm limit should be observed when testing the device in order to prevent any rotational injury to the knee during testing. To prevent injury to the patient due to loss of circulation in the foot, the boot’s pressure will constantly be monitored and feedback from the patient considered. Examination of the device will be performed before every use to assure the measurement apparatus is in working order. This will eliminate down time for the patient during the test and reduce fatigue.

A failure mode for this device includes fatigue of the operator throughout the duration of the test. The lower leg should be leveled in two planes before the initialization of the test, in order to obtain consistent data. The sensors should be continually monitored throughout the test in order to ensure they are adhered properly. A loose sensor may result in inaccurate data. If the inflatable lining of the boot is punctured, there will be a loss of stability of the boot, allowing the foot to move around in the boot. A puncture of the boot’s lining can be fixed by repairing the puncture or replacing the lining.

The development of the rotational stability measurement device will be completed according to the Design Controls 21 CFR 820.30. The device will be designed to be compliant with all FDA regulations and validated, and these steps will be recorded according to document control regulations.

D.2 Data Collection and Processing

Moment data from the UFS and rotation data from the Nest of Birds will be processed using a Matlab program which will display moment as a function of rotation. The knee will first be externally rotated, until a force target of 10Nm is reached, then internally rotated until the same force target is reached. In this fashion the entire range of motion of the knee in internal/external rotation can be measured. Also, the variability between patients will be minimized by leveling the lower leg in two planes to establish a standard starting position.

If a constant moment is applied to the knee, recorded moment of the knee joint should increase slowly, in a linear fashion, until knee laxity is eliminated and stress begins to be applied to the ligaments of the knee joint. This would result in a rapid increase in recorded moment. Corresponding rotations of the knee would show a linear increase in rotation as a constant force is applied, until the value plateaus when the ‘slack’ is taken up in the knee and the ligaments begin to take on the force applied. These patterns would be expected upon preliminary testing of the device.

D.3. Validation and Accuracy

Instructions will be given to the user upon purchase as to proper fitting of the brace so to eliminate movement of the shank within the brace during testing. The UFS will be calibrated by applying known loads in all 6 DOF before testing. The Nest of Birds system will be initialized before each test.

To validate the device, several preliminary studies were conducted to determine the repeatability of the measurements. The goal is to ensure that range of motion is recorded to within ±1°. Changes will be made to the design of the device and the protocol of the tests according to the results of these validation studies.

D.4. Preliminary Studies

Once the device is validated, preliminary testing can occur on healthy subjects. The comfort and safety of the patients will be an item of interest during these preliminary trials. If the patient is uncomfortable or placed into an awkward position during the tests, then appropriate design changes will be made. Proper training will be provided to medical personnel utilizing the device to ensure safe force applications and minimal movement in other directional planes. Phase II studies will then determine the range of knee motion to diagnose ACL deficient patients or to evaluate the efficacy of ACL reconstruction techniques.

An initial study was done to determine the repeatability of the rotation measurements obtained using the device. Four subjects were selected and underwent a clinical knee evaluation to determine the health of the knee and suitability for the study. The subject was comfortably fitted with the boot and brace, and the Nest of Birds sensors were placed on the femur, tibia, and boot. The lower leg was leveled at the horizontal to create a standard start position for each subject. 10Nm internal and external moments were applied to the knee by a clinician and the range of motion recorded by the Nest of Birds. Each subject was tested for five trials at the knee flexion angles of 0, 45 and 90°. A sample output from the Matlab program is shown in Figure 2. It can be seen that the 10Nm moment target was reached, resulting in a 18° external rotation and 31° internal rotation.

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Figure 2: Sample Matlab output

Preliminary results indicate that the device is repeatable but some adjustments will need to be made. The plots of the path of motion for the four subjects can be seen in Appendix 1. It can be seen that at the extremes of knee rotation, the moment target is reached. The moment increases sharply as the knee reaches its rotational limit.

Table 1 shows the range of motion in degrees for the four subjects.

|Flexion Angle |Subject 1 |Subject 2 |Subject 3 |Subject 4 |

|0 degrees |40.75 ± 1.7 |62.20 ± 1.4 |43.40 ± 1.5 |64.40 ± 1.9 |

|45 degrees |49.60 ± 1.8 |52.00 ± 1.0 |31.60 ± 0.9 |57.00 ± 0.7 |

|90 degrees |37.00 ± 1.6 |37.00 ± 0.7 |27.20 ± 1.5 |54.20 ± 1.8 |

Table 1: Average Range of Motion (degrees) ± 1 Standard Deviation

The results show that measurements are within ±2° which is close to our goal of ±1°. Some changes can be made to improve the device, potentially giving more repeatable data. The weight of the device and specifically the boot should be considered, as operator fatigue may have come into play when testing. Some suggestions to eliminate the bulkiness of the design would be to shorten the torque bar so the operator may use one hand to apply the moment while supporting the leg with the other. Also, a shorter version of the Pneumatic Walker may also improve the results by reducing unnecessary bulk.

D.5. Impact and Significance of Design

Validation of this device will have a revolutionary effect on the ACL reconstruction procedure. It will institute new standards in the field and new procedures in the operating room. The measurement device prototype will be further evaluated by the Orthopedic/Ferguson Lab to potentially be brought to market by Aircast. The physical arrangement of the brace can also be modified to include an apparatus to test anterior-posterior laxity. This incorporation of both the rotation and translational stability devices into one will make devices such as the KT 1000 and the Rolimeter obsolete.

E. References

1.) ACL Reconstruction,

.

2.) 2000 IKDC Knee Forms,

3.) Bellier, Guy, et al. 2004. Double-Stranded Hamstring Graft for Anterior Cruciate Ligament Reconstruction. The Journal of Arthoscopic and Related Surgery, 20(8): 890-984.

4.) Georgoulis, Anastasios, et al. 2003. Three-Dimensional Tiobiofemoral Kinematics of the Anterior Cruciate Ligament-Deficient and Reconstructed Knee during Walking. The American Journal of Sports Medicine, 31(1): 75-79.

5.) Guardamanga, Luca, et al. 2004. Double-band reconstruction of the ACL using a synthetic implant: a cadaveric study of knee laxity. Journal of Orthopaedic Science, 9: 372-379.

6.) Kurz, Max, et al. 2004. The effect of anterior cruciate ligament reconstruction on lower extremity relative phase dynamics during walking and running. Knee Surgery, Sports Traumatology, Arthroscopy.

7.) MEDmetric KT1000 Arthrometer Information. MEDmetric Corporation.

8.) Schuster, Andreas, et al. 2004. A New Mechanical Testing Device for Measuring Anteroposterior Knee Laxity. The American Journal of Sports Medicine, 32(7): 1731-1735.

9.) Tohyama, Harukazu, et al. 2003. Anterior drawer test for acute anterior talofibular ligament injuries of the ankle. The American Journal of Sports Medicine, 31(2): 226-232.

10.) Woo, Savio L.-Y., et al. 2000. Injury and repair of ligaments and tendons. Annual Review of Biomedical Engineering, 2: 83-118.

11.) Yamamoto, Yuji, et al. 2004. Knee Stability and Graft Function After Anterior

Cruciate Ligament Reconstruction. The American Journal of Sports Medicine, 32(8): 1825-1832.

12.) Pneumatic Walker Product Information, Aircast Corp,

13.) Markolf, KL, et al. Stiffness and laxity of the knee--the contributions of the

supporting structures. The Journal of Bone and Joint Surgery, Vol 58, Issue 5

583-594, 1976.

14.) Forster, I, et al. Is the KT-1000 knee ligament arthrometer reliable? The Journal of

Bone and Joint Surgery, Vol 71, 843-847, 1989.

F. Gantt Chart

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G. Appendix 1

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