Biomechanical Soft-Tissue Research Influences Product Development

An important aspect of orthopaedic surgery is the management of soft tissues, which include cartilage, ligaments, tendons, and muscles. Biomechanical research quantifies structural and mechanical characteristics of such tissues as well as their functional demands in healthy, diseased, injured, repaired, or healed states. In addition to the mechanical and structural complexity of soft tissues, one of the unique challenges is addressing the viscoelastic nature of these tissues. This article will discuss the role of soft-tissue biomechanics research on the rotator cuff, tendon repair, and product innovation.

From a biomechanics perspective, to understand any tissue and its function around the shoulder, two types of testing are typically performed: isolated tissue testing and structural analysis of the joint. Isolated tissue testing quantifies the biomechanical characteristics of the specific tissue being studied. This information provides the tissue’s mechanical (material) properties as well as structural properties of specific tissue constructs. Meanwhile, functional testing of the joint elucidates specific mechanical demands of individual anatomic structures as well as functional demands of the joint. Specifically, the analysis quantifies the contribution of each anatomic structure of interest and functional capabilities.

With repaired tissues, the goal is to create a strong biomechanical construct for early mobilization and optimal healing. To accomplish this, surgeons have developed various fixation devices, sutures, and unique repair strategies, along with appropriate instrumentation.

In isolated tissue construct testing, the biomechanical characteristics of normal and repaired soft tissues such as the rotator cuff are evaluated and compared. (Fig. 1). Materials-testing principles are employed, and the tissue construct is subjected to both cyclic loading and loading to failure. For each phase of cyclic loading, two important biomechanical parameters are typically determined: linear stiffness and hysteresis. The linear stiffness of the tissue construct is defined as the slope of the linear portion of the load-elongation curve in force (N) per deformation (mm).

Intuitively, the tissue construct’s stiffness is its ability to resist deformation. The hysteresis of the tissue construct is the area between the loading and unloading curves on the load-elongation curve. Hysteresis represents energy dissipated throughout the tissue construct during each loading and unloading cycle. This energy can be dissipated in many ways, including suture anchor–bone slippage, knot slippage, tissue fiber alignment, etc. Hysteresis will usually decrease with each cycle during cyclic loading as well as with increasing stiffness of a construct. When a tissue construct is loaded to failure, the investigators can quantify the following important biomechanical parameters:

• Stiffness of the tissue construct is its ability to resist deformation.
• Yield load and deformation are the point at which the load/elongation curve deviates from linearity (i.e., when stiffness begins to decrease). The yield point represents the transition point between elastic and plastic deformation of the construct. Prior to reaching yield load, the construct is in the elastic range, where all load and deformation are recoverable. However, once the yield point is passed, permanent plastic deformation occurs, resulting in injury.
• Ultimate load and deformation point are where tissue construct failure occurs.
• Energy absorbed is calculated at both yield load and ultimate points by determining the area under the load–elongation curve. This parameter represents the tissue construct’s ability to absorb energy before an injury or failure.

Gap formation is the migration of a tendon edge away from its footprint during loading. It is commonly measured with video digitizing software to track the movement of the tendon edge as the tissue construct is loaded. This process allows for evaluation of the structural integrity of repaired tissues, as well as determination of the strength of fixation. With rotator cuff repair constructs, this parameter is also used to define clinical failure.

Clinically, using isolated tissue construct testing, many new rotator cuff repair strategies have been developed and evaluated, including single row, double row, trans-osseus equivalent, and many other variations. For fixation, there is a new shift toward all-suture anchors from hard anchors. Biomechanical studies have demonstrated the fundamental differences between these two types of anchors. All-suture anchors can be just as strong as hard anchors and more bone preserving; however, they are susceptible to creep elongation, as the fixation strength depends on expansion of the sutures, where the deployment of the anchor can be variable.

Functional testing
Functional testing is performed to quantitatively assess the complex interplay between the passive and active stabilizing mechanisms during function. This process permits an evaluation of joint function and the repair/reconstruction’s effectiveness. The challenges here are the complexity of the anatomy, the nonhomogeneous nature of soft tissue, and viscoelastic behavior.

To perform a functional test for a shoulder, a custom testing apparatus that permits control of all shoulder degrees of freedom as well as simulation of functional muscle groups in a cadaveric shoulder is typically used. Specifically, the system consists of a large loading frame, a 6-degree-of-freedom load cell, servomotors equipped with load cells, a kinematic measurement system, and an intra-articular contact measuring system.

Specifically, to simulate muscle function, a tendon clamp-cable-pulley system is used to apply muscle forces to the rotator cuff, the anterior and posterior portions of the deltoid, pectoralis major, and latissimus dorsi on a cadaveric shoulder. A computer with customized software controls the servomotors attached to the tendon clamps as well as acquisition of force data for each muscle. Glenohumeral position is measured with a motion-tracking or 3D digitizing system. Glenohumeral joint reaction force is measured with a 6-degree-of-freedom load cell, and intra-articular contact characteristics are measured with a pressure sensor. The system also permits external loading to simulate forces during various shoulder activities (Fig. 2).

Clinically, using a shoulder testing apparatus, various shoulder pathologies and repair/reconstruction strategies have been investigated to determine effectiveness. For example, the superior capsule reconstruction (SCR) procedure has been evaluated extensively for treatment of irreparable massive rotator cuff tears to prove its efficacy. Thereafter, using the SCR concept, many new procedures such as biceps SCR and anterior cable reconstruction have been developed and evaluated.

In summary, biomechanics research plays an important role in orthopaedic product innovation. It helps us to understand tissue- and joint-specific characteristics, as well as evaluate healthy, diseased, injured, or repaired anatomic regions both at the tissue and joint levels. Likewise, biomechanical research can help to advance current knowledge on individual tissue structures as well as joint function by testing the efficacy of new, innovative technologies, devices, and repair strategies.

Thay Q. Lee, PhD, is the director of research at Congress Medical Foundation in Pasadena, California.