Prostaglandin E(2), collagenase, and cell death responses depend on cyclical load magnitude in an explant model of tendinopathy.

Tendinopathy is a significant clinical problem that can result from repetitive activity. While the precise etiology of this condition remains unclear, the cellular response to cyclical loading is believed to have a contributory role to the pathology of tendinopathy. This study examined the short-term biochemical response of avian flexor digitorum profundus tendon to repetitive cyclic loadings of varying magnitude. An in vitro tendon explant model was utilized to apply four levels of haversine tensile stress (peak stress of 0, 3, 12, and 18 MPa) at 1.0 Hz, 8 hr/day for 3 days. The 12 and 18 MPa levels were known to cause significant mechanical damage based on previous work. Tissue media was recovered and analyzed for prostaglandin E(2) (PGE(2)), lactate dehydrogenase (LDH, measure of cell death), and collagenase levels. Tissue samples were recovered and analyzed for cell viability, total collagen, and sulfated glycosaminoglycan content. Collagenase, LDH, and PGE(2) levels were found to be influenced by loading magnitude (p < 0.05) with higher levels being present at higher load magnitudes. Varying cyclical load magnitude caused minimal compositional changes as collagen content and glycosaminoglycan did not change. These results indicate that elevated cyclical mechanical loading of tendon quickly results in altered biochemical tissue responses indicative of tissue injury. More sustained cyclical loading over time may be required for these initial responses to induce more dramatic tissue changes as observed in clinical tendinopathy.

Distributing a fixed amount of cyclic loading to tendon explants over longer periods induces greater cellular and mechanical responses.

Tendon overuse injuries are a major source of clinical concern. Cyclic loading causes material damage and induces biochemical responses in tendon. The purpose of this study was to examine the biochemical and biomechanical tendon response after applying cyclical loading over varying durations. Avian flexor digitorum profundus tendons were loaded (3 or 12 MPa) to a fixed number of cycles across either 1 or 12 days in vitro. The tendon response evaluations included biomechanical data gathered during loading and subsequent failure testing. Evaluations also included cellular viability, cell death, and proteoglycan, collagen, collagenase, and prostaglandin E(2) (PGE(2)) content measurements obtained from tissue specimens and media samples. Significant strains (up to 2%) accumulated during loading. Loading to 12 MPa significantly reduced maximum stress (33% and 27%) and energy density (42% and 50%) when applied across 1 or 12 days, respectively. Loading to 3 MPa also caused a 40% reduction in energy density, but only when applied across 12 days. Cell death and collagenase activity increased significantly with increasing magnitude and duration. However, no differences occurred in cell viability or collagen content. Glycosaminoglycan content increased 50% with load magnitude, while PGE(2) production increased 2.5-fold with loading magnitude and 11-fold with increased duration. Mechanical fatigue-induced mechanical property changes were exhibited by the tendons in response to increased loading magnitude across just 1 day. However, when the same loading was applied over a longer period, most outcomes were magnified substantially, relative to the short duration regimens. This is presumably due to the increased response time for the complex cellular response to loading. A key contributor may be the inflammatory mediator, PGE(2), which exhibited large magnitude and duration dependent increases to cyclic loading.

A tissue explant system for assessing tendon overuse injury.

Tendon overuse injuries are common athletic and occupational problems. When studying mechanisms that cause these injuries, inherent complexities associated with controlling in vivo loading necessitates alternative approaches such as in vitro organ culture. Current devices for loading explants in organ culture, whether custom-built or commercial, have various deficiencies in their loading capability, control mechanism and strain assessment. To overcome these shortcomings, an advanced tissue loading device with video strain analysis capabilities was developed for investigating overuse injuries and its performance/calibration were evaluated. Two tests were used to assess the ability of the system to create and monitor mechanical changes with overuse. Overuse loading significantly increased strains and decreased strength, showing the ability of this system to create and monitor tissue damage. Furthermore, the device design allows for its use in a standard incubator. Coupled with custom loading and data collection programs, this system is suitable for long-term overuse injury studies.

Thermal microdebridement does not affect the time zero biomechanical properties of human patellar tendons.

BACKGROUND: Thermal microdebridement for the treatment of chronic tendinopathy has recently been introduced. The effect of thermal microdebridement on the biomechanical properties of human tendons, however, remains unknown. HYPOTHESIS: Thermal microdebridement does not affect the biomechanical properties of human patellar tendons in a cadaveric model at the time of initial treatment. STUDY DESIGN: Controlled laboratory study. METHODS: The central 15 mm of 12 matched, human (mean age, 71 years; 8 male, 4 female), fresh-frozen patellar tendons was divided into 3 equal 5-mm specimens. The treatment group (n = 12) underwent thermal microdebridement with a radiofrequency probe. A sham treatment group (n = 12) underwent insertion of a deactivated probe. The control group (n = 12) underwent no treatment. After treatment, each specimen was tested to failure in a servo-hydraulic materials testing machine at an elongation rate of 3 mm/s. One-way repeated measures analysis of variance was used to determine differences between groups. RESULTS: No significant difference in ultimate stress at failure, elastic modulus, strain energy density, or strain at maximum load was found between the groups. The ultimate stress at failure for the treatment, sham, and control groups was 61.0, 66.7, and 63.0 MPa, respectively (P = .653), and the strain at maximum load was 0.12, 0.11, and 0.09, respectively (P = .279). CONCLUSIONS: Thermal microdebridement does not affect the biomechanical properties of cadaveric human patellar tendons at the time of initial treatment. CLINICAL RELEVANCE: It may be safe to proceed with aggressive rehabilitation after thermal microdebridement of the patellar tendon. However, the results in this cadaveric model should be interpreted with caution. Additional studies using an in vivo model will be required to completely assess the effects of thermal microdebridement on the biomechanical properties of human patellar tendons.

Mechanical response of tendon subsequent to ramp loading to varying strain limits.

OBJECTIVE: To determine the in vitro elastic limit of avian flexor tendons utilizing more current methodologies. DESIGN: Assess the mechanical changes between subfailure and subsequent failure ramps at various loading levels. BACKGROUND: Currently accepted values of elastic strain limits were determined utilizing older methodologies. Consequently, reported values are fairly small and imply matrix damage occurs with small strains. METHODS: Flexor tendons were loaded in vitro, to various subfailure strain levels between 1% and 14%, allowed to rest for 5 min, then taken to failure. Suture markers, across the midsubstance, and cryo-grip displacement were monitored for strain using a video strain analysis system and a linear variable displacement transducer, respectively. The mechanical changes between the various subfailure and failure ramp loadings were assessed. RESULTS: Varying strains of subfailure ramp loading did not influence (P>0.05) the ultimate tensile failure strength, elastic modulus, strain at failure, or strain energy density of tendons. In addition, residual strain after subfailure loading was not significant, nor was it influenced by the level of the subfailure loading. CONCLUSIONS: Tendon behaves elastically under ramp loading to significantly higher strains (nearly failure) than previously reported (4%). RELEVANCE: This study has found strain thresholds required to cause matrix damage to be significantly higher than previously thought, implying that matrix changes to acute loading events are more a result of the cellular response to the loading event. The clinical relevance is that the clinician may have a greater opportunity to prevent matrix changes than once thought by biochemically altering the cellular response.