Do Prophylactic Knee Braces Protect the Knee Against Impacts or Tibial Moments? An In Vitro Multisensory Study

Abstract

Background: Knee braces are prescribed by physicians to protect the knee from various loading conditions during sports or after surgery, even though the effect of bracing for various loading scenarios remains unclear.

Purpose: To extensively investigate whether bracing protects the knee against impacts from the lateral, medial, anterior, or posterior directions at different heights as well as against tibial moments.

Study design: Controlled laboratory study.

Methods: Eight limb specimens were exposed to (1) subcritical impacts from the medial, lateral, anterior, and posterior directions at 3 heights (center of the joint line and 100 mm inferior and superior) and (2) internal/external torques. Using a prophylactic brace, both scenarios were conducted under braced and unbraced conditions with moderate muscle loads and intact soft tissue. The change in anterior cruciate ligament (ACL) strain, joint acceleration in the tibial and femoral bones (for impacts only), and joint kinematics were recorded and analyzed.

Results: Bracing reduced joint acceleration for medial and lateral center impacts. The ACL strain change was decreased for medial superior impacts and increased for anterior inferior impacts. Impacts from the posterior direction had substantially less effect on the ACL strain change and joint acceleration than anterior impacts. Bracing had no effect on the ACL strain change or kinematics under internal or external moments.

Conclusion: Our results indicate that the effect of bracing during impacts depends on the direction and height of the impact and is partly positive, negative, or neutral and that soft tissue absorbs impact energy. An effect during internal or external torque was not detected.

Clinical relevance: Bracing in contact sports with many lateral or medial impacts might be beneficial, whereas athletes who play sports with rotational moments on the knee or anterior impacts may be safer without a brace.

Keywords: ACL strain; acceleration; brace; kinematic analysis; knee.

Figure 1.

(A) Radiograph of the implanted sensors and muscle force application. (B) Schematic images of the implanted sensors. Two Bowden cables (1) were pulled through the soft tissue, with one of the cables anchored in the posterior tibia to simulate the hamstring muscle (6) and the other crimped to a perforated plate (2) and sewn to the quadriceps tendon to simulate the quadriceps. A borehole was drilled in both the posterior femoral and the tibial bones, and an acceleration sensor was cemented into each hole (3 and 5). Notchplasty was performed, and the differential variable reluctance transducer strain sensor was pinned in the anteromedial bundle of the anterior cruciate ligament (4).

Figure 2.

Test setup: The limb (1) was mounted in the testing rig, and the weight (2) on the lever arm (3) was accelerated by the pneumatic actuator (4), creating a frontal impact 100 mm above the center of the joint line. The optical markers (5) were tracked with a 3-dimensional camera system (6).

Figure 3.

Sequence of a measurement: (A) Scheme of the test setup for a medial impact at a height of 100 mm above the center of the joint line. (B-D) High-speed recording (240 fps) of the impact. It can be seen in D how the weight is mounted with low friction on the lever arm as the position relative to the lever arm in B has changed.

Figure 4.

Example of an impact from the lateral direction at the height of the center of the joint line and in the braced condition. The dashed line marks the time of impact. (A) Raw data of the femoral and tibial acceleration sensors. (B) The 3-dimensional plot shows that maximal acceleration occurs in the direction of the impact. (C) The anterior cruciate ligament in the braced condition is more relaxed than in the unbraced condition, as indicated by an offset of approximately 0.3%. (D) Only adduction is affected by a lateral impact, whereas flexion and internal rotation are unchanged.

Figure 5.

Results of all parameters for lateral and medial impacts of 1 J at the height of the center of the joint line and 100 mm above and below at 30° of flexion. Green indicates the unbraced condition and blue the braced condition. Acceleration within the bone was reduced in some cases, whereas the effect on the anterior cruciate ligament strain change was only significant for high medial impacts. Bracing did not influence kinematics. Data are displayed as the median and interquartile range. *P < .05.

Figure 6.

Results of all parameters for anterior and posterior impacts of 1 J at the height of the center of the joint line and 100 mm above and below at 30° of flexion. Green indicates the unbraced condition and blue the braced condition. Acceleration within the bone was only reduced for low anterior impacts. The anterior cruciate ligament strain change was increased for low anterior impacts on the shin. Bracing did not influence kinematics. Data are displayed as the median and interquartile range. *P < .05.

Figure 7.

Results of all parameters for internal and external tibial moments of 5 N·m in 30° and 60° of flexion. Green indicates the unbraced condition and blue the braced condition. Neither the anterior cruciate ligament strain change nor kinematics was significantly altered by bracing. Data are displayed as the median and interquartile range.