Joseph J. Sarver

Effect of altered matrix proteins on quasilinear viscoelastic properties in transgenic mouse tail tendons

AbstractTendons have complex mechanical behaviors that are viscoelastic, nonlinear, and anisotropic. It is widely held that these behaviors are provided for by the tissues composition and structure. However, little data are available to quantify such structure–function relationships. This study quantified tendon mechanical behaviors, including viscoelasticity and nonlinearity, for groups of mice that were genetically engineered for altered extracellular matrix proteins. Uniaxial tensile stress-relaxation experiments were performed on tail tendon fascicles from the following groups: eight week old decorin knockout, eight week old reduced type I collagen, three week old control, and eight week old control. Data were fit using Fungs quasilinear viscoelastic model, where the model parameters represent the linear viscoelastic and nonlinear elastic response. The viscoelastic properties demonstrated a larger and faster stress relaxation for the decorin knockout and a smaller and slower stress relaxation for the three week control. The elastic parameter, A, in the eight week control group was significantly greater than in the collagen reduction and three week control groups. This study provides quantitative evidence for structure–function relationships in tendon, including the role of proteoglycan in viscoelasticity. Future studies should directly correlate composition and structure with tendon mechanics for the design and evaluation of tissue-engineered constructs or tendon repairs.

The Effect of Postoperative Passive Motion on Rotator Cuff Healing in a Rat Model

BACKGROUND Surgical repairs of torn rotator cuff tendons frequently fail. Immobilization has been shown to improve tissue mechanical properties in an animal model of rotator cuff repair, and passive motion has been shown to improve joint mechanics in animal models of flexor tendon repair. Our objective was to determine if daily passive motion would improve joint mechanics in comparison with continuous immobilization in a rat rotator cuff repair model. We hypothesized that daily passive motion would result in improved passive shoulder joint mechanics in comparison with continuous immobilization initially and that there would be no differences in passive joint mechanics or insertion site mechanical properties after four weeks of remobilization. METHODS A supraspinatus injury was created and was surgically repaired in sixty-five Sprague-Dawley rats. Rats were separated into three postoperative groups (continuous immobilization, passive motion protocol 1, and passive motion protocol 2) for two weeks before all underwent a remobilization protocol for four weeks. Serial measurements of passive shoulder mechanics (internal and external range of motion and joint stiffness) were made before surgery and at two and six weeks after surgery. After the animals were killed, collagen organization and mechanical properties of the tendon-to-bone insertion site were determined. RESULTS Total range of motion for both passive motion groups (49% and 45% of the pre-injury values) was less than that for the continuous immobilization group (59% of the pre-injury value) at two weeks and remained significantly less following four weeks of remobilization exercise. Joint stiffness at two weeks was increased for both passive motion groups in comparison with the continuous immobilization group. At both two and six weeks after repair, internal range of motion was significantly decreased whereas external range of motion was not. There were no differences between the groups in terms of collagen organization or mechanical properties. CONCLUSIONS In this model, immediate postoperative passive motion was found to be detrimental to passive shoulder mechanics. We speculate that passive motion results in increased scar formation in the subacromial space, thereby resulting in decreased range of motion and increased joint stiffness. Passive motion had no effect on collagen organization or tendon mechanical properties measured six weeks after surgery.

Exercise following a short immobilization period is detrimental to tendon properties and joint mechanics in a rat rotator cuff injury model.

Rotator cuff tears are a common clinical problem that can result in pain and disability. Previous studies in a rat model showed enhanced tendon to bone healing with postoperative immobilization. The objective of this study was to determine the effect of postimmobilization activity level on insertion site properties and joint mechanics in a rat model. Our hypothesis was that exercise following a short period of immobilization will cause detrimental changes in insertion site properties compared to cage activity following the same period of immobilization, but that passive shoulder mechanics will not be affected. We detached and repaired the supraspinatus tendon of 22 Sprague‐Dawley rats, and the injured shoulder was immobilized postoperatively for 2 weeks. Following immobilization, rats were prescribed cage activity or exercise for 12 weeks. Passive shoulder mechanics were determined, and following euthansia, tendon cross‐sectional area and mechanical properties were measured. Exercise following immobilization resulted in significant decreases compared to cage activity in range of motion, tendon stiffness, modulus, percent relaxation, and several parameters from both a structurally based elastic model and a quasi‐linear viscoelastic model. Therefore, we conclude that after a short period of immobilization, increased activity is detrimental to both tendon mechanical properties and shoulder joint mechanics, presumably due to increased scar production.

Methods for Quasi-Linear Viscoelastic Modeling of Soft Tissue: Application to Incremental Stress-Relaxation Experiments

The quasi-linear viscoelastic (QLV) model was applied to incremental stress-relaxation tests and an expression for the stress was derived for each step. This expression was used to compare two methods for normalizing stress data prior to estimating QLV parameters. The first and commonly used normalization method was shown to be strain-dependent. Thus, a second normalization method was proposed and shown to be strain-independent and more sensitive to QLV time constants. These analytical results agreed with representative tendon data. Therefore, this method for normalizing stress data was proposed for future studies of incremental stress-relaxation, or whenever comparing stress-relaxation at different strains.

After rotator cuff repair, stiffness—but not the loss in range of motion—increased transiently for immobilized shoulders in a rat model

Although rotator cuff repair is often successful at relieving pain, the repaired insertion site frequently fails. Mechanical properties of the repair improved when the shoulder was immobilized in an animal model, but joint stiffness and range of motion were not evaluated. The objective of this study was to measure rotational mechanics before and after shoulders were immobilized after cuff injury and repair, not immobilized after cuff injury and repair, and immobilized without injury and repair. Humeral rotation was significantly less 4 and 8 weeks after injury and repair but did not decrease significantly when the injured and repaired shoulder was immobilized. Rotational stiffness increased significantly 4 and 8 weeks after injury and repair and was significantly greater at 4, but not 8, weeks when the injured and repaired shoulders were immobilized. This study demonstrated that the increase in joint stiffness caused by immobilizing an injured and repaired shoulder was transient and, therefore, does not outweigh the long-term benefits of immobilization on improved tendon to bone healing.

Restoration of Anterior-Posterior Rotator Cuff Force Balance Improves Shoulder Function in a Rat Model of Chronic Massive Tears

The rotator cuff musculature imparts dynamic stability to the glenohumeral joint. In particular, the balance between the subscapularis anteriorly and the infraspinatus posteriorly, often referred to as the rotator cuff “force couple,” is critical for concavity compression and concentric rotation of the humeral head. Restoration of this anterior–posterior force balance after chronic, massive rotator cuff tears may allow for deltoid compensation, but no in vivo studies have quantitatively demonstrated an improvement in shoulder function. Our goal was to determine if restoring this balance of forces improves shoulder function after two‐tendon rotator cuff tears in a rat model. Forty‐eight rats underwent detachment of the supraspinatus and infraspinatus. After four weeks, rats were randomly assigned to three groups: no repair, infraspinatus repair, and two‐tendon repair. Quantitative ambulatory measures including medial/lateral forces, braking, propulsion, and step width were significantly different between the infraspinatus and no repair group and similar between the infraspinatus and two‐tendon repair groups at almost all time points. These results suggest that repairing the infraspinatus back to its insertion site without repair of the supraspinatus can improve shoulder function to a level similar to repairing both the infraspinatus and supraspinatus tendons. Clinically, a partial repair of the posterior cuff after a two‐tendon tear may be sufficient to restore adequate function. An in vivo model system for two‐tendon repair of massive rotator cuff tears is presented.

Effect of Age and Proteoglycan Deficiency on Collagen Fiber Re-Alignment and Mechanical Properties in Mouse Supraspinatus Tendon

Collagen fiber realignment is one mechanism by which tendon responds to load. Re-alignment is altered when the structure of tendon is altered, such as in the natural process of aging or with alterations of matrix proteins, such as proteoglycan expression. While changes in re-alignment and mechanical properties have been investigated recently during development, they have not been studied in (1) aged tendons, or (2) in the absence of key proteoglycans. Collagen fiber re-alignment and the corresponding mechanical properties are quantified throughout tensile mechanical testing in both the insertion site and the midsubstance of mouse supraspinatus tendons in wild type (WT), decorin-null (Dcn(-/-)), and biglycan-null (Bgn(-/-)) mice at three different ages (90 days, 300 days, and 570 days). Percent relaxation was significantly decreased with age in the WT and Dcn(-/-) tendons, but not in the Bgn(-/-) tendons. Changes with age were found in the linear modulus at the insertion site where the 300 day group was greater than the 90 day and 570 day group in the Bgn(-/-) tendons and the 90 day group was smaller than the 300 day and 570 day groups in the Dcn(-/-) tendons. However, no changes in modulus were found across age in WT tendons were found. The midsubstance fibers of the WT and Bgn(-/-) tendons were initially less aligned with increasing age. The re-alignment was significantly altered with age in the WT tendons, with older groups responding to load later in the mechanical test. This was also seen in the Dcn(-/-) midsubstance and the Bgn(-/-) insertion, but not in the other locations. Although some studies have found changes in the WT mechanical properties with age, this study did not support those findings. However, it did show fiber re-alignment changes at both locations with age, suggesting a breakdown of tendons ability to respond to load in later ages. In the proteoglycan-null tendons however, there were changes in the mechanical properties, accompanied only by location-dependent re-alignment changes, suggesting a site-specific role for these molecules in loading. Finally, changes in the mechanical properties did not occur in concert with changes in re-alignment, suggesting that typical mechanical property measurements alone are insufficient to describe how structural alterations affect tendons response to load.

The effect of overshooting the target strain on estimating viscoelastic properties from stress relaxation experiments.

BACKGROUND Tendons mechanical behaviors have frequently been quantified using the quasi-linear viscoelastic (QLV) model. The QLV parameters are typically estimated by fitting the model to a single-step stress relaxation experiment. Unfortunately, overshoot of the target strain occurs to some degree in most experiments. This has never been formally investigated even though failing to measure, minimize, or compensate for overshoot may cause large errors in the estimation of parameters. Therefore, the objective of this study was to investigate the effect of overshoot on the estimation of QLV parameters. METHOD OF APPROACH A simulated experiment was first performed to quantify the effect of different amounts of overshoot on the estimated QLV parameters. Experimental data from tendon was then used to determine if the errors associated with overshoot could be reduced when a direct fit is used (i.e., the actual strain history was used in the curve fit). RESULTS We found that both the elastic and viscous QLV parameters were incorrectly estimated if overshoot was not properly accounted for in the fit. Furthermore, the errors associated with overshoot were partially reduced when overshoot was accounted for using a direct fit. CONCLUSIONS A slow ramp rate is recommended to limit the amount of overshoot and a direct fit is recommended to limit the errors associated with overshoot, although other approaches such as adjusting the control system to limit overshoot could also be utilized.

Plasticity of Muscle Architecture After Supraspinatus Tears

STUDY DESIGN Controlled laboratory study. OBJECTIVES To measure the architectural properties of rat supraspinatus muscle after a complete detachment of its distal tendon. METHODS Supraspinatus muscles were released from the left humerus of 29 Sprague-Dawley rats (mass, 400-450 g), and the animals were returned to cage activity for 2 weeks (n=12), 4 weeks (n=9), or 9 weeks (n=8), before euthanasia. Measurements of muscle mass, pennation angle, fiber bundle length (sarcomere number), and sarcomere length permitted calculation of normalized fiber length, serial sarcomere number, and physiological cross-sectional area. RESULTS Coronal oblique sections of the supraspinatus confirmed surgical transection of the supraspinatus muscle at 2 weeks, with reattachment by 4 weeks. Muscle mass and length were significantly lower in released muscles at 2 weeks, 4 weeks, and 9 weeks. Sarcomere lengths in released muscles were significantly shorter at 2 weeks but not different by 4 weeks. Sarcomere number was significantly reduced at 2 and 4 weeks, but returned to control values by 9 weeks. The opposing effects of smaller mass and shorter fibers produced significantly smaller physiological cross-sectional area at 2 weeks, but physiological cross-sectional area returned to control levels by 4 weeks. CONCLUSIONS Release of the supraspinatus muscle produced early radial and longitudinal atrophy of the muscle. The functional implications of these adaptations would be most profound at early time points (particularly relevant for rehabilitation), when the muscle remains smaller in cross-sectional area and, due to reduced sarcomere number, would be forced to operate over a wider range of the length-tension curve and at higher velocities, all adaptations resulting in compromised force-generating capacity. These data are relevant to physical therapy because they provide tissue-level insights into impaired muscle and shoulder function following rotator cuff injury.

Transient Decreases in Forelimb Gait and Ground Reaction Forces Following Rotator Cuff Injury and Repair in a Rat Model

Due to inadequate healing, surgical repairs of torn rotator cuff tendons often fail, limiting the recovery of upper extremity function. The rat is frequently used to study rotator cuff healing; however, there are few systems capable of quantifying forelimb function necessary to interpret the clinical significance of tissue level healing. We constructed a device to capture images, ground reaction forces and torques, as animals ambulated in a confined walkway, and used it to evaluate forelimb function in uninjured control and surgically injured/repaired animals. Ambulatory data were recorded before (D-1), and 3, 7, 14, 28 and 56 days after surgery. Speed as well as step width and length were determined by analyzing ventral images, and ground reaction forces were normalized to body weight. Speed averaged 22+/-6 cm/s and was not affected by repair or time. Step width and length of uninjured animals compared well to values measured with our previous system. Forelimbs were used primarily for braking (-1.6+/-1.5% vs +2.5+/-0.6%), bore less weight than hind limbs (49+/-5% vs 58+/-4%), and showed no differences between sides (49+/-5% vs 46+/-5%) for uninjured control animals. Step length and ground reaction forces of the repaired animals were significantly less than control initially (days 3, 7 and 14 post-surgery), but not by day 28. Our new device provided uninjured ambulatory data consistent with our previous system and available literature, and measured reductions in forelimb function consistent with the deficit expected by our surgical model.