Link to Figures Show Figures
	Article Text Figures References
	Send Link Send HTML Article

Reduction in Anterior Cruciate Ligament Load and Tibiofemoral Rotation under Applied Axial Rotation: A Surrogate Model Study of the Efficacy of a New Knee Derotation Brace Concept

Paul R. Mathewson, PhD
Richard M. Greenwald, PhD

ABSTRACT A new, adjustable, bidirectional derotation brace, developed to limit tibiofemoral rotation under applied torsional loading, was tested under both static and dynamic loading conditions using an instrumented knee surrogate. In braced versus unbraced comparisons (n = 4), the brace significantly reduced surrogate-anterior cruciate ligament (ACL) load by an average of 18% and tibiofemoral rotation by an average of 25% when physiologically relevant levels of applied femoral torque (12-70 N-m) were statically applied to the knee joint in a controlled laboratory setting. In addition, reductions in surrogate-ACL load of 34% and 27% with the brace were recorded for two levels of dynamic impact loading. The effectiveness of this new brace concept in limiting surrogate-ACL load and tibiofemoral rotation under applied axial torsion may be relevant for ACL injury prevention, and has not, to our knowledge, been demonstrated in currently available knee braces.

Knee ligament injuries are a major health problem in the United States and elsewhere. It has been estimated that 80,000 to 100,000 anterior cruciate ligament (ACL) injuries occur each year in the United States alone.^1,2 Such injuries represent significant losses of both health care resources and athletic participation. Over the past several decades, efforts to reduce the incidence or recurrence of knee ligament injuries have focused on the use of post-hinge-strap/shell design braces to restrict abnormal knee motions thought to be involved in soft tissue injuries to the knee. Post-hinge-strap/shell braces were designed primarily to address varus/valgus forces and anterior tibial translations in the knee joint. Although some studies have supported the use of bracing,^3-7 other studies designed to validate the effectiveness of functional and prophylactic post-hinge-strap/shell braces have thus far failed to confirm their protective capabilities^8-15. In general, these braces resist varus/valgus impact and/or anterior translation at low applied loads but are judged to be ineffective at controlling such forces under physiologically relevant loads.

Knee ligament injuries often occur by way of multiple knee-loading factors and may result from contact and/or noncontact interactions. A primary goal of the rigid metal posts used in the post-hinge-strap/shell brace design is to resist varus/valgus forces applied to the knee. Such forces often occur as the result of direct contact. However, the majority of injuries, especially those involving the ACL, are noncontact injuries.^16-18 These noncontact injuries usually involve some degree of axial rotation in the knee joint and/or anterior tibial translation.^17,19-21

The exigency to restrict axial rotation in the knee joint was recognized early in the development of knee bracing and led to the development of the Lenox Hill Derotation Brace.^22,23 While this brace was of post-hinge-strap construction, it incorporated an antirotation strap in the design. Subsequent testing to confirm the ability of this brace to restrict axial rotation and/or anterior tibial translation proved inconclusive.^3,8,13,23-25 France et al.²⁶ and Cawley²⁷ opined that post-hinge-strap/shell braces are, by design, unable to effectively restrict axial rotation in the knee joint.

The biomechanical basis for ACL injury as a result of axial rotation in the knee has been described previously. Fetto and Marshall²⁸ described the "pivot shift" referring to a clinical instability of the knee joint. They point out the general agreement that the ACL acts as a stabilizer of the knee against both anterior and internal-rotation displacement and note that excessive internal rotation of an extended or hyperextended knee is a common mechanism resulting in ACL rupture. Markolf et al.²⁹ demonstrated in cadaver studies that internal tibial torque applied to an anteriorly loaded knee produced dramatic increases in force on the ACL, particularly at or near full extension. These authors judged the combination of anterior loading and internal tibial torque to be the most dangerous in terms of potential ACL injury. In addition, they showed that overall risk of ACL injury from varus/valgus moment applied in combination with internal tibial torque was similar to the risk from internal tibial torque alone. Wascher et al.³⁰ showed in cadaver studies using specially designed load-transducers that applied internal tibial torque produced the greatest force in the ACL when the knee joint was fully extended. Greenwald et al.³¹ also used a cadaver model to show that increased internal tibial torque significantly increased ACL strain in an extended knee. Beynnon et al.⁸ described internal tibial torque as a primary mechanism of ACL injury. Although of somewhat less concern, external tibial rotation also increases ACL load at or near full extension.²⁹ McNair et al.¹⁷ reported the mechanism of injury in 20 subjects with isolated ACL ruptures. Of 19 subjects who could recall the circumstances of their injury, 15 reported that the injury occurred at or near full extension with either excessive internal or external tibial rotation. Thus, there remains a clear need for an orthopedic appliance capable of effectively restraining axial rotation in the knee joint, especially at or near full knee extension.

The brace concept reported in this preliminary proof-of-concept study was developed to overcome the limitations inherent in current post-hinge-strap/shell brace design in restricting tibiofemoral rotation under applied torsional loading. Tests of brace function were conducted using an instrumented knee surrogate model previously used for lateral impact testing of knee braces.^10,12,32,33 The parametric experimental protocol reported herein was designed to test two hypotheses: 1) that the derotation brace would reduce surrogate-ACL load under applied torsional loading compared with the unbraced surrogate knee and 2) that the derotation brace would reduce the degree of tibiofemoral rotation in the surrogate under the same torsional loading conditions compared with the unbraced surrogate knee. This preliminary study was intended to test the functionality of the brace design in restricting axial rotation in the knee joint. No conclusions were drawn regarding the possible effectiveness of the brace on an intact human joint.

METHODS

The derotation brace consists of an undersleeve (Bioskin Underskin, Large, 41430; Cropper Medical, Tucson, AZ) positioned against the skin about the knee joint, and a set of fabric bracing members wound in a circumferentially spiraling manner about the limb, attaching to the undersleeve at points above and below the joint (Figure 1 ). Hook and loop material (Velcro) was incorporated onto the outer surface of the undersleeve as well as on the inner surface of the fabric bracing members. An outersleeve of the same Bioskin material covers the applied bracing members. The bracing members are made from Spandura (style 941158L/946160; H. Warshow & Sons, Inc., New York, NY) a strong, lightweight fabric material having unidirectional stretch characteristics. Hook and loop material was attached to the ends of each bracing member. Four bracing members were wound in a spiraling manner around the surrogate limb in each direction, beginning at the upper thigh, passing the knee joint at the medial and lateral condyles, and extending to mid-shaft of the tibia. Thus wound, the bracing members covered the soft tissue areas of the thigh and upper calf but left the anterior and posterior of the knee joint exposed. A single examiner applied the brace components to the surrogate limb, attaching each bracing member to the undersleeve using hook and loop material. The resistance to axial rotation is a user-adjustable function of the degree to which the bracing members are tightened when the brace is applied. Although the bracing members should pass the joint at the medial and lateral condyles to facilitate normal knee flexion, the very precise positioning of the brace, as required by current post-hinge-strap/shell brace designs, is less important for proper function in this brace design. The flexible nature of all brace components allows for ease of fit to virtually any limb shape and size.

The instrumented knee surrogate model used in this study was designed for parametric analysis of knee braces under valgus loading conditions.^10,12,32,33 This surrogate was validated using valgus-loaded human cadaver data.³² Although not validated to an intact human knee joint, this surrogate was constructed with anatomically correct geometry and was expected to provide a close approximation of basic knee joint kinematics. The authors indicated that the dynamic loaded response of the surrogate was within 2% of normalized cadaver data for direct valgus loads.^12,33

The surrogate femur and tibia were molded in aluminum from actual human bones. The knee joint consisted of a Delrin patella, Delrin menisci, and wire cable "ligaments" oriented anatomically to represent the ACL, medial collateral ligament (MCL), lateral collateral ligament (LCL), and posterior cruciate ligament (PCL). Each cable was instrumented with a strain gauge calibrated to record individual cable loads. A spring aligned with the cable allowed for pre-tensioning of the surrogate ligament cables. An additional two cable/spring units represented the quadriceps and hamstring muscle groups and were attached to approximate anatomical locations for these muscle groups. For tests reported in this prefatory study, the knee joint was positioned at approximately 5° of flexion. The surrogate "ankle" was fixed to the impact frame to prevent motion of the tibia. The rubberized-foam soft tissue covering, placed over the instrumented limb, approximated the compliance and contour of a normal human leg.³²

An electromagnetic three-dimensional positioning system (Polhemus Fastrack; Polhemus, Inc., Winooski, VT) measured internal/external rotation of the femur relative to the fixed tibia under applied load (Figure 1 ). The transmitter was mounted on a stationary frame such that the internal/external rotation axis of the transmitter was aligned with the long axis of the tibia. The receiver was mounted rigidly to the proximal femur near the hip joint. Preliminary testing was performed on the unbraced surrogate to establish repeatability.

An initial preload of 125 N was applied to each of the four ligament cable systems (ACL, PCL, MCL, and LCL) at the beginning of the test sequence. Quadriceps tendon tension was set such that the knee remained at 5° of flexion with no significant anterior tibial translation. Hamstring tension was fixed at 100 N.

Joint compression was simulated by adding a 150 N weight to the surrogate, pushing vertically down on the femur from above (Figure 1 ). A zero-load reading was taken on each ligament cable before torsional loading and again after the applied loading sequence to determine pre- and post-test repeatability. Loads were applied at a distance of 17 cm perpendicular to the long axis of the femur to cause an external rotation of the femur relative to the fixed tibia. Six loads ranging from 70 to 413 N (12-70 N-m torque) were applied to the femur. Tibiofemoral rotation was unconstrained except for the restraining forces of the ligaments and friction in the meniscal bearings because of joint compression.

For each parametric analysis, testing of the braced condition was followed by testing of the unbraced condition. After testing the braced condition, the outer sleeve was rolled down to the ankle and each bracing member was carefully detached. The undersleeve was left in place. Tibiofemoral rotation was monitored to ensure a zero starting point and the testing procedure was repeated for the unbraced condition. Each set of braced and unbraced data was evaluated individually.

Static and dynamic loading tests were performed. Static loads were applied via a cable with dead weights to produce an applied femoral torque on the surrogate knee resulting in external rotation of the femur relative to the fixed tibia (Figure 1 ). After reaching static equilibrium, data including loads on the ACL, LCL, MCL, and PCL were collected at 10 Hz for 5 seconds. Tibiofemoral rotation of the femur from the initial neutral position was recorded at each load interval using the Polhemus Fastrack. A total of four parametric tests (braced vs. unbraced) were performed.

Dynamic loading was evaluated by dropping a 70 N weight from two heights, 15 and 30 cm, to produce an instantaneous torque on the test system and a rapid rotation of the femur relative to the tibia. For these dynamic loading tests, three parametric tests were performed at each drop height. The brace was reapplied between tests of each data set. Data were collected at 1,000 Hz.

A repeated-measures analysis of variance was performed using brace condition (on/off) and applied load as the main effects. Measured variables were ligament load and tibiofemoral rotation. The null hypothesis was that there would be no effect of the brace on ligament load or tibiofemoral rotation at any individual applied load. A significance level of p < .05 was set a priori. Post hoc tests using the same level of significance were performed when necessary.

RESULTS

Repeatability tests demonstrated that the load measured by the surrogate-ACL ligament system was independent of the order of the different statically applied loads. Repeatability was established (±4%) by applying a single load of 370 N (63 N-m torque) three separate times after brace removal and reapplication. Because application of the brace required unbolting the surrogate "foot" to externally rotate the tibia, it was decided to order the testing by evaluating the braced condition first, followed by the unbraced condition.

The derotation brace had a significant effect in decreasing both surrogate-ACL load (p < 0.05) (Figure 2 , Table 1 ) and tibiofemoral rotation (p < 0.05) (Figure 3 , Table 2 ) compared with the unbraced condition at all levels of applied femoral torque. For the static-load tests, surrogate-ACL load was reduced by 18% ± 7% (-3.6% to 32%) in the braced condition compared with the unbraced condition (Table 1 ). For applied torque greater than 12 N-m, these differences were highly significant (p <0.005). A similar reduction of 25% ± 19% (-6.4% to 80%) was observed for tibiofemoral rotation in the braced condition compared with the unbraced condition (Table 2 ). The brace maintained the ability to restrict these variables throughout the range of applied loads. Under dynamic impact loading, surrogate-ACL load was decreased by 34% and 27% (braced vs. unbraced) for the 15- and 30-cm drop heights, respectively (Figure 4 ). Although data for PCL, MCL, and LCL loads were also collected and demonstrated similar trends, only the ACL data is reported here.

DISCUSSION

This preliminary study evaluates the efficacy of a new knee derotation brace design in reducing ligament loading and tibiofemoral rotation under applied torsional load compared with the unbraced condition using an instrumented knee surrogate model. Although the surrogate used to study brace function was not specifically designed for rotational studies and was not validated to an intact human joint, the surrogate provides what we believe to be an appropriate platform for evaluating the functional attributes of the new brace under the parametric experimental conditions described. The parametric testing protocol used was intended to demonstrate the functionality of the brace prototype in a rotational mode. That is, we intended to test whether, under constant surrogate conditions, the brace could significantly reduce surrogate-ACL load and tibiofemoral rotation compared with the unbraced surrogate under physiologically relevant applied axial loads. No conclusions regarding possible protective affects in an intact human knee joint are claimed or inferred from these data.

The derotation brace significantly reduced both the load on the surrogate-ACL and tibiofemoral rotation compared with the unbraced condition for a range of physiologically relevant applied femoral torque. The surrogate knee flexion angle was set near full extension. This condition was chosen because of the prevalence of noncontact ACL injuries and the fact that the majority of such injuries occur with the knee at or near full extension or in hyperextension. ACL strain has been shown to be most sensitive to internal/external rotation and anterior-posterior translation at or near full extension.^{16,17,21,29,31} The frequency of ACL injuries in sports and their physiological mechanism point to the need for an effective means of reducing tibiofemoral rotation, anterior tibial translation, and their attendant forces, which lead to injury of the ligaments of the knee. Load conditions used in these tests would be indicative of a rotation of the torso and the hips relative to a fixed tibia, as might happen when an athlete in motion plants the foot and twists the upper body relative to the fixed foot. Likewise, the loading regimen simulates conditions arising from a situation in which a skier suffers a forward-twisting fall with weight applied to the body of the ski.

The use of the instrumented surrogate model, previously used for lateral impact studies, was supported by the repeatability of the data in the unbraced condition. There seemed to be no structural interference with the menisci or with the test frame when loads were applied to induce femoral rotation. To minimize any effect of retensioning the ligament cables between tests, and because minimal loading to the surrogate knee resulted in minor fluctuations in the ligament tension values, no retensioning to a value of 125 N was performed. Preliminary tests on the surrogate demonstrated that the load on the ACL and the other ligaments at higher applied loads was repeatable (within 5%) independent of the initial tension on the ACL ligament pretensioned within 30% of the original 125 N applied before testing.

The parametric static-load tests demonstrated an average 18% decrease in surrogate-ACL load and an average 25% decrease in tibiofemoral rotation in the braced versus unbraced condition over the range of applied loads. Variations in the ACL loads and tibiofemoral rotations among the four parametric trials is likely because of different tensions resulting from the application of the bracing members in setting up the braced condition. A negative percentage was observed for one value of ACL load and one of tibiofemoral rotation when applying the relatively light 12 N-m torque (Table 1 and Table 2 ). This negative value indicates that this applied torque did not overcome the restraining force of the applied bracing members compared with the unbraced surrogate knee in that paired test. In these tests, the bracing member tension was neither measured nor controlled, although the same examiner applied the bracing members and attempted to achieve similar bracing member pretension.

As noted previously, increased internal tibial rotation correlates with increased ACL strain and increased ACL strain is a precondition for potential ACL rupture. The data reported here showing significant decrease in surrogate-ACL load and tibiofemoral rotation suggest that this new derotation bracing system may offer some protective capabilities by preventing excessive rotation in these loading situations. It is recognized, however, that the absolute degree of tibiofemoral rotation that causes ACL injury is not known and is probably a function of many variables, including knee flexion angle, muscle contraction, joint compression, and individual anatomy and physiology. Additional studies are planned to further define the functionality of this bracing concept and its possible efficacy on an intact joint.

Many noncontact ACL injuries are described as the result of rapid, abrupt, or sudden changes in body direction.^16,17 These injuries clearly occur at high rates of rotation and applied load. It is therefore important that a bracing device be able to respond to rapidly applied loads as well as static applied forces. For this reason, the ability of the brace to respond appropriately to dynamically applied loads, causing an instantaneous torque on the surrogate knee, was investigated. The derotation brace was shown to effectively reduce surrogate-ACL load compared with the unbraced condition for these dynamically applied loads. The brace produced a 34% decrease in ACL load at the 15-cm drop height (Figure 4A ) and a 27% decrease in ACL load at the 30-cm drop height (Figure 4B ). Tibiofemoral rotation data were not collected for these dynamic trials because of the limited frequency response of the Polhemus system.

In addition to axial rotation in the joint, anterior tibial translation is also an important factor in noncontact knee injuries.^17,21,28,29 Thus, in addition to restricting internal/external tibiofemoral rotation, an effective brace will also limit anterior tibial translation (ATT). To provide a preliminary assessment of this capability, ATT was measured using the KT-2000 Arthrometer (Medmetric Corp., San Diego, CA) on several subjects under clinical conditions. Test conditions were not optimized. A single examiner applied the derotation brace. Data for a single ACL-deficient subject (Figure 5 ) show a reduction in anterior tibial translation of ~2.5 mm in the braced condition compared with the unbraced condition. The force required to initiate and continue translation was also greater in the braced condition compared with the unbraced condition. Although limited, these preliminary data are useful in that they suggest that the derotation brace may also effectively restrain anterior tibial translation. Further studies of this capability are anticipated.

Because the knee is a complex joint and can be injured by a variety of possible mechanisms, bracing of the joint is necessarily a complex goal. It is important to consider as many approaches to protecting the joint as may be practical. For example, it has been suggested that functional braces (of post-hinge-strap/shell design) may provide some benefit through a number of different modalities³⁴; that is, their benefit may derive not only from biomechanical reinforcement of the knee structure, but also by providing an enhanced proprioceptive sense. Whether current-design functional/prophylactic braces have such a proprioception-enhancing effect has been the subject of several reports with conflicting conclusions. Cook et al.³⁵ indicated that post-hinge-strap/shell functional braces might provide a stabilizing proprioceptive effect. In contrast, a report by Branch et al.³⁶ concluded that results from an electromyographic study failed to show a proprioceptive effect with such bracing. Beynnon et al.³⁴ studied the effect of ACL disruption, functional bracing, and a neoprene sleeve on knee proprioception by measuring the threshold to detection of passive knee motion. The use of a functional brace or a neoprene sleeve did not significantly improve proprioception on the injured knee.

The muscles crossing the knee, including the quadriceps, hamstring, and gastrocnemius muscles, are important stabilizers of the knee joint, acting through a mechanism involving dynamic compression of the joint. This stabilizing effect has been demonstrated to decrease both anterior tibial translation as well as rotations in the joint.^37-39 Wojtys et al.,¹⁵ referring to the dynamic joint compression system formed by these muscles, also suggested that this muscle group could improve the ability of a brace to stabilize the knee joint. They further suggest that proprioceptive augmentation could, theoretically, improve dynamic control in unstable knees.

The derotation brace reported here represents a new concept which may address both the biomechanical and proprioceptive goals of knee bracing. The brace has no hard structural elements, no hinges and allows for normal knee flexion. Brace function is activated by axial rotation in the joint. Construction of this derotation brace is such that as a rotational load is applied (i.e., in the case of the knee, the femur rotates relative to the tibia), the diameter circumscribed by the circumferentially wound bracing members decreases. This dynamic response to axial rotation results in a compressive force on the soft tissues of the limb, as well as on the knee joint itself, thereby restraining further rotation. Because the bracing members can be applied to spiral in both a clockwise and counterclockwise direction, rotation of the joint in either or both directions will produce this compressive rotation-restraining force. The extent of such delimiting rotation is user-adjustable, depending on the bracing member pre-tension when initially applied to the limb. The concept of stabilizing the knee joint through joint compression is a relevant consideration in understanding the mechanism by which the present derotation brace functions. The greater the pretension when the brace is applied, the more compression about the limb and joint as axial rotation increases. This compression about the joint restrains further rotation and may also lend stability to the joint by enhancing the muscle compression noted by Wojtys et al.¹⁵

Further, the compressive force exerted by the bracing members on the soft tissues surrounding the knee joint is dynamic in nature. That is, as rotation increases, soft tissue compression increases. Correspondingly, as rotation returns to a neutral position, compression decreases to the initial level of pre-tension. The wearer of the brace can readily perceive these changes in compressive force. Thus, this brace design, in addition to reducing ACL load and tibiofemoral rotation at physiologically relevant loads, may also provide the compressive forces and sense of rotational proprioception which have been suggested as being beneficial to enhancing joint stability.

LIMITATIONS

This study represents the first of several steps in the complex process of qualifying the effectiveness of a novel derotation brace as an additional means for preventing knee injuries. No claims of efficacy in humans are made or implied as a result of this study. The positive results thus far encourage additional research using human subjects that replicate clinical conditions. There are several important limitations in this study. The use of a surrogate to quantify the human condition is at best an approximation. Soft tissue compliance, muscle loading, and joint compression are all variables that affect ligament loading. However, surrogates do allow effective parametric comparison between trial conditions, such as braced and unbraced. The anatomic surrogate used was originally validated against cadaver data for valgus impact. The anatomic positions of the cable ligaments, the use of Delrin menisci, and the application of quadriceps and hamstring loads to stabilize the joint improve the relationship between the surrogate and human conditions but represent only one combination of loading conditions and tissue parameters.

The soft tissue covering used was identical to that used in the lateral impact brace studies reported by France et al.^10,12,32,33 We are not aware of any artificial soft tissue covering that completely replicates the dynamic stiffness and volume of the leg muscles. The surrogate soft tissue represents a relatively firm model for the soft tissue, and is applicable for parametric studies. Cawley²⁷ demonstrated that soft tissues in the leg have little resistance to tangential loading, thus severely limiting the ability of post-hinge-strap/shell braces to control axial rotation in the knee joint. The brace design reported herein represents a significant departure from this earlier design model. The winding bracing members substantially cover the thigh and upper calf, and apply increased compressional force (rather than tangential) on the soft tissue with increasing tibiofemoral rotation. The flexible nature of the brace construct is designed to accommodate changes in muscle shape with flexion angle and contraction. Additional studies are required to validate the effectiveness of the design in resisting rotation with changes in these variables.

Loading parameters chosen for this study were based on literature data in cadavers showing that failure of one or more knee ligaments in axial rotation occur between 28 and 43 N-m of applied axial torque.⁴⁰ The quasi-static and dynamic loading conditions were intended to cover a range of physiologically relevant loading conditions above these reported failure loads. The results showed a positive effect of the brace in reducing rotation and ligament loading over 2 orders of magnitude of loading rate. This again stimulates additional research on the rate dependence of the loading along with the other physiological and anatomical variables discussed above.

The goal of this study was to parametrically evaluate the ability of this new brace design to resist rotation and to reduce surrogate ligament loading in a simulated situation that is known to be a mechanism of ACL injury.^28,29,31,34 With that demonstrated, these data encourage additional investigation to evaluate different experimental conditions, to expand the research to clinical trials to measure the dynamic ability of the brace to resist rotation, and, if possible, to measure ACL loading with and without the brace. Other variables to be studied include effectiveness over the range of knee flexion angles, effect of brace strap tightness on efficacy and on user comfort, and dynamic movement of the brace on the leg during exercise.

In recent years, the American Academy of Orthopedic Surgeons has concluded that there is no direct evidence that knee braces could prevent knee injuries. This conclusion is based on the results of previous studies using post-hinge-strap/shell brace designs. However, this lack of success in the past should not discourage future innovation in brace design. Research to validate new concepts should proceed as have earlier brace investigations: first by evaluating them in the laboratory and then, with encouraging results, expanding the scope of the research to test efficacy under actual clinical conditions. Our continuing research will pursue the goal of evaluating the effectiveness of the derotation brace in clinical applications.

CONCLUSIONS

Preliminary testing of a new derotation prototype bracing system demonstrated that the brace is effective in reducing surrogate-ACL load as well as tibiofemoral rotation under physiologically relevant applied torsional loading under controlled laboratory conditions. Though additional studies are needed to elucidate the possible effectiveness of the brace on intact human joints, these results are significant in demonstrating that effective control of axial rotation in human joints may be practical with this brace design. Reduction in the magnitude of these variables may have an impact on the likelihood of suffering an ACL or other knee injury. The combination of these indicators, including decreased ACL load, decreased tibiofemoral rotation, and decreased anterior tibial displacement under the test conditions described suggest that this new bracing concept may offer a unique solution to certain modes of joint injury and instability that are not effectively addressed by current post-hinge-strap/shell brace designs. In addition, the muscle and joint compression achieved as a result of the bracing mechanism, as well as a sense of rotational proprioception unique to this brace design, may also enhance joint stability.

ACKNOWLEDGMENTS

We thank Dr. George Milliken of the Department of Statistics, Kansas State University (Manhattan, Kansas) for assistance in the statistical evaluation of the data presented in this report. The instrumented surrogate leg was provided by dj Orthopedics (formerly Smith and Nephew DonJoy). The work described in this article was conducted at the Orthopedic Biomechanics Institute, Salt Lake City, Utah.

PAUL R. MATHEWSON, PhD, is affiliated with GrayMatter Inc., Park City, UT.

RICHARD M. GREENWALD, PhD, is affiliated with Simbex LLC, Lebanon, NH. Correspondence to: Paul R. Mathewson, Ph.D., GrayMatter, Inc., 7726 N. Buckboard Dr., Park City, UT 84098; e-mail: paulm@xmission.com

References:

1. Griffin LY, Agel J, Albohm MJ, Arendt EA, Dick RW, Garrett WE, Garrick JG, Hewett TE, Huston L, Ireland ML, Johnson RJ, Kibler WB, Lephart S, Lewis JL, Lindenfeld TN, Mandelbaum BR, Marchak P, Teitz CC, Wojtys EM. Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. J Am Acad Orthop Surg 2000;8:141-150.
2. Huston LJ, Greenfield ML, Wojtys EM. Anterior cruciate ligament injuries in the female athlete: potential risk factors. Clin Orthop 2000;372:50-63.
3. Bassett GS, Fleming BW. The Lenox Hill brace in anterolateral rotary instability. Am J Sports Med 1983;11:345-348.
4. Vailas JC, Pink M. Biomechanical effects of functional knee bracing: practical implications. Sports Med 1993;15:210-218.
5. Wojtys EM, Huston LJ. Neuromuscular performance in normal and anterior cruciate ligament-deficient lower extremities. Am J Sports Med 1994;22:89-104.
6. Torry MR, O'Connor DD, Kedrowski JJ, Sterett WI, Steadman JR. Knee stability controlled by hamstrings and functional knee brace. J Prosthet Orthot 2001;13:90-96.
7. Fleming BC, Beynnon B, Engstrom B, Peura G. Functional knee bracing reduces strains on the ACL during weightbearing and non-weightbearing conditions. In: Proceedings of the 46th Annual Meeting of the Orthopaedic Research Society; 2000 March 12-15; Orlando, Florida. Rosemont (IL): Orthopaedic Research Society; 2000:476.
8. Beynnon BD, Pope MH, Wertheimer CM, Johnson RJ, Fleming BC, Nichols CE, Howe JG. The effect of functional knee-braces on strain on the anterior cruciate ligament in vivo. J Bone Joint Surg Am 1992;74:1298-1312.
9. Cawley PW, France EP, Paulos LE. The current state of functional knee bracing research: a review of the literature. Am J Sports Med 1991;19:226-233.
10. France EP, Paulos LE. In vitro assessment of prophylactic knee brace function. Clin Sports Med 1990;9:823-841.
11. Liu SH, Lunsford T, Gude S, Vangsness CT. Comparison of functional knee braces for control of anterior tibial displacement. Clin Orthop Rel Res 1994;303:203-210.
12. Paulos LE, Cawley PW, France EP. Impact biomechanics of lateral knee bracing: the anterior cruciate ligament. Am J Sports Med 1991;19:337-342.
13. Wojtys EM, Goldstein SA, Redfern M. A biomechanical evaluation of the lenox hill knee brace. Clin Orthop Rel Res 1987;220:179-184.
14. Wojtys EM, Loubert PV, Samson SY, Viviano DM. Use of a knee brace for control of tibial translation and rotation. J Bone Joint Surg Am 1990;72:1323-1329.
15. Wojtys EM, Kothari SU, Huston LJ. Anterior cruciate ligament functional brace use in sports. Am J Sports Med 1996;24:539-546.
16. Delfico AJ, Garrett WE. Mechanisms of injury of the anterior cruciate ligament in soccer players. Clin Sports Med 1998;17:779-785.
17. McNair PJ, Marshall RN, Matheson JA. Important features associated with acute anterior cruciate ligament injury. N Z Med J 1990;901:537-9.
18. Noyes FR, Mooar PA, Matthews DS, Butler DL. The symptomatic anterior cruciate-deficient knee. Part I: the long-term functional disability in athletically active individuals. J Bone Joint Surg Am 1983;65:154-162.
19. Ebstrup JF, Bolsen-Moller F. Anterior cruciate ligament injury in indoor ball games. Scand J Med Sci Sports 2000;10:114-116.
20. Jarvinen M, Natri A, Laurila S, Kannus P. Mechanisms of anterior cruciate ligament ruptures in skiing. Knee Surg Sports Traumatol Arthrosc 1994;2:224-228.
21. Kirkendall DT, Garrett WE. The anterior cruciate ligament enigma: injury mechanisms and prevention. Clin Orthop 2000;372:64-68.
22. Nicholas JA. The five-one reconstruction for the anteromedial instability of the knee. J Bone Joint Surg Am 1973;55:899-922.
23. Nicholas JA. Bracing the anterior cruciate ligament deficient knee using the lenox hill derotation brace. Clin Orthop 1983;172:137-142.
24. Branch TP, Hunter RE. functional analysis of anterior cruciate ligament braces. Clin Sports Med 1990;9:771-797.
25. Colville MR, Lee CL, Ciullo JV. The Lenox Hill brace: an evaluation of effectiveness in treating knee instability. Am J Sports Med 1988;14:257-261.
26. France EP, Cawley PW, Paulos LE. Choosing functional knee braces. Clin Sports Med 1990;9:743-750.
27. Cawley PW. Postoperative knee bracing. Clin Sports Med 1990;9:763-769.
28. Fetto JF, Marshall JL. Injury to the anterior cruciate ligament producing the pivot-shift sign. An experimental study on cadaver specimens. J Bone Joint Surg Am 1979;61:710-714.
29. Markolf KL, Burchfield DM, Shapiro MM, Shepard MF, Finerman GA, Slauterbeck JL. combined knee loading states that generate high anterior cruciate ligament forces. J Orthop Res 1995;13:930-935.
30. Wascher DC, Markolf KL, Shapiro MS, Finerman GA. direct in vitro measurement of the forces in the cruciate ligaments. Part I: The effect of multiplane loading in the intact knee. J Bone Joint Surg Am 1993;75:377-386.
31. Greenwald RM, Swanson SC, McDonald TR. Towards the creation of a new release envelope for protecting the knee in alpine skiing. Sportorthopadie-Sporttraumatologie 1999;15:1-8.
32. France EP, Paulos LE, Jayaraman G, Rosenberg TD. The biomechanics of lateral knee bracing. Part II: Impact response of the braced knee. Am J Sports Med 1987;15:430-438.
33. Paulos LE, France EP, Rosenburg TD, Jayaraman, G, Abbott PJ, Jaen J. The biomechanics of lateral knee bracing. Part I: Response of the valgus restraints to loading. Am J Sports Med 1987;15:419-429.
34. Beynnon BD, Ryder SH, Konradsen L, Johnson RJ, Johnson K, Renstrom PA. The effect of anterior cruciate ligament trauma and bracing on knee proprioception. Am J Sports Med 1999;27:150-155.
35. Cook FF, Tibone JE, Redfern FC. A dynamic analysis of a functional brace for anterior cruciate ligament insufficiency. Sports Med 1989;17:519-524.
36. Branch TP, Hunter R, Reynolds P. Controlling anterior tibial displacement under static load: a comparison of two braces. Orthopedics 1988;11:1249-1252.
37. Goldfuss AJ, Morehouse CA, LeVeau BF. Effect of muscular tension on knee stability. Med Sci Sports 1973;5:267-271.
38. Markolf KL, Kochan A, Amstutz HC. Measurement of knee stiffness and laxity in patients with documented absence of anterior cruciate ligament. J Bone Joint Surg Am 1984;66:242-253.
39. Wang CJ, Walker PS. Rotary laxity in the human knee joint. J Bone Joint Surg Am 1974;56:161-170.
40. Berns GS, Hull ML, Patterson HA. Strains within the anterior cruciate and medial collateral ligaments of the knee at loads causing failure. In: Johnson RJ, Mote CD Jr, and Zelcer CD, ed. Skiing Trauma and Safety: Ninth International Symposium, ASTM STP 1182. Philadelphia: American Society for Testing Materials; 1993:89-110.