Improved strength of early versus late supraspinatus tendon repair: a study in the rabbit.

HYPOTHESIS: The optimal timing for surgical repair of the supraspinatus (SSP) tendon after full-substance tear has not been established. The objectives of this prospective investigation of SSP tendon repair delayed by 1, 2, or 3 months followed by a 3-month postoperative course were to (1) determine the site of failure, (2) measure the tensile strength and stiffness, and (3) assess the ability of computed tomography to predict mechanical strength. MATERIALS AND METHODS: We transected 1 SSP tendon in 36 rabbits and then repaired it with transosseous sutures after a delay of 1, 2, or 3 months. We compared the results with 36 intact shoulders from 18 age-matched control rabbits. RESULTS: Experimental specimens failed at the tendon (n = 26) more often than at the enthesis (n = 10) (P < .05). The mean peak loads to failure 3 months after repair delayed by 1 month and delayed by 2 months were significantly greater than their respective control values (P < .05 for both); there was no difference after a delay of 3 months. There was no association between the presence of hypoattenuation on computed tomography and repair strength (P > .05). CONCLUSIONS: Our findings indicate better mechanical results with earlier repair (1 or 2 months) after SSP tendon than after a delay of 3 months. Early surgical repair may lower the risk of tendon retear.

Bone marrow fat accumulation after 60 days of bed rest persisted 1 year after activities were resumed along with hemopoietic stimulation: the Women International Space Simulation for Exploration study.

Immobility in bed and decreased mobility cause adaptations to most human body systems. The effect of immobility on fat accumulation in hemopoietic bone marrow has never been measured prospectively. The reversibility of marrow fat accumulation and the effects on hemopoiesis are not known. In the present study, 24 healthy women (age: 25-40 yr) underwent -6 degrees head-down bed rest for 60 days. We used MRI to noninvasively measure the lumbar vertebral fat fraction at various time points. We also measured hemoglobin, erythropoietin, reticulocytes, leukocytes, platelet count, peripheral fat mass, leptin, cortisol, and C-reactive protein during bed rest and for 1 yr after bed rest ended. Compared with baseline, the mean (+/-SE) fat fraction was increased after 60 days of bed rest (+2.5+/-1.1%, P<0.05); the increase persisted 1 yr after the resumption of regular activities (+2.3+/-0.8%, P<0.05). Mean hemoglobin levels were significantly decreased 6 days after bed rest ended (-1.36+/-0.20 g/dl, P<0.05) but had recovered at 1 yr, with significantly lower mean circulating erythropoietin levels (-3.8+/-1.2 mU/ml, P<0.05). Mean numbers of neutrophils and lymphocytes remained significantly elevated at 1 yr (+617+/-218 neutrophils/microl and +498+/-112 lymphocytes/microl, both P<0.05). These results constitute direct evidence that bed rest irreversibly accelerated fat accumulation in hemopoietic bone marrow. The 2.5% increase in fat fraction after 60 days of bed rest was 25-fold larger than expected from historical ambulatory controls. Sixty days of bed rest accelerated by 4 yr the normal bone marrow involution. Bed rest and marrow adiposity were associated with hemopoietic stimulation. One year after subjects returned to normal activities, hemoglobin levels were maintained, with 43% lower circulating erythropoietin levels, and leukocytes remained significantly elevated across lineages. Lack of mobility alters hemopoiesis, possibly through marrow fat accumulation, with potentially wide-ranging clinical consequences.

The ability of ultrasonography, magnetic resonance imaging and bone mineral densitometry to predict the strength of human Achilles' tendons.

OBJECTIVE: To assess the value of ultrasonography (US), magnetic resonance imaging (MRI), and bone mineral densitometry (BMD) in evaluating human Achilles' tendon strength. DESIGN: Cross-sectional observational study. SETTING: Tertiary care hospital. PARTICIPANTS: Ninety-eight Achilles' tendons from 49 consecutive cadavers (26 men and 23 women with a mean age of 66.6 years) undergoing hospital autopsy were assessed. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Tendon dimensions on US and MRI, and T1-weighted optical density were measured. Areas of hypodensity, hyperdensity, calcification, and heterogeneity were identified on US. The BMD of each calcaneus was recorded. The tendons were mechanically tested to determine peak load at failure. RESULTS: Sixteen patients (32.7%, 27 tendons) had abnormalities in 1 or both tendons on US and/or MRI (17 on US, 17 on MRI). Fifty-seven tendons (58%) ruptured in their midsubstance, at an average peak load of 4722+/-990N. Tendons with and without abnormalities on imaging had similar strengths (P>.05). Calcaneal BMD correlated weakly with peak load at failure (r=.21, P<.05). CONCLUSIONS: The prevalence of Achilles' tendons abnormalities on US or MRI was 32.7% in our study group. Abnormalities on clinical imaging (US or MRI) were not predictive of the load at failure. Therefore, tendons with imaging abnormalities are not necessarily weaker, and one cannot predict the likelihood of rupture based on imaging results. Further, higher-powered studies could explore the ability of BMD to detect minimal clinically important differences and to predict Achilles' tendon weakness.

Mechanical alterations of rabbit Achilles' tendon after immobilization correlate with bone mineral density but not with magnetic resonance or ultrasound imaging.

OBJECTIVE: To assess the usefulness of magnetic resonance imaging (MRI), ultrasound (US) imaging, or bone mineral density (BMD) in predicting the mechanical properties of immobilized rabbit Achilles' tendons. DESIGN: Experimental study. SETTING: Basic university laboratory. ANIMALS: Twenty-eight rabbits. INTERVENTIONS: Twelve rabbits had 1 hindlimb casted for 4 weeks and 10 rabbits were casted for 8 weeks. Contralateral legs and 12 normal hindlimbs served as controls. MAIN OUTCOME MEASURES: Achilles' tendon dimensions on MRI and US, T1- and T2-signal intensities on MRI, classification of abnormalities on MRI and US; BMD of the calcaneus with dual-energy x-ray absorptiometry. Biomechanic measures consisted of peak load, stiffness, and stress. Imaging variables were correlated with biomechanic alterations. RESULTS: Immobilized Achilles' tendons were weaker and showed decreased mechanical stress compared with their contralateral legs and controls (all P<.05). MRI and US revealed larger Achilles' tendons after immobilization. However, neither increased MRI nor US signal abnormality was found. BMD was lower in immobilized calcanei and larger in contralateral legs than controls. Only BMD correlated with both the decreased peak load (R2=.42, P<.05) and stress (R2=.54, P<.05) of immobilized Achilles' tendon. CONCLUSIONS: This study established weakened mechanical properties of immobilized Achilles' tendons. BMD of the calcaneus, but not MRI and US, was predictive of the mechanical alterations in immobilized Achilles' tendons. BMD may be a useful biomarker to monitor disease and recovery in Achilles' tendons.

Dual cryogenic fixation for mechanical testing of soft musculoskeletal tissues.

Mechanical testing of soft musculoskeletal structures like tendons and ligaments are essential to medical advances. A long-standing limitation for testing these structures in isolation has been the ability to solidly fix both ends of the tendon. Cryogenic fixation technology was leveraged into the development of a dual cryogenic fixation (DCF) device. Results of the study show that the DCF allows tendons to be tested in isolation, at physiologic temperatures, with excellent reproducibility.

Supraspinatus tendon repair into a bony trough in the rabbit: mechanical restoration and correlative imaging.

Recurrence of tears is a common complication after rotator cuff surgery. Retearing seems to occur early after surgery and may be attributed to too early or too vigorous exercises. We found no experimental data correlating the strength of the rotator cuff early after surgery and imaging. Our objectives were to measure the peak load to failure of rabbit supraspinatus tendon-bone constructs at early times postoperatively, to determine their mode of failure, and to determine whether computed tomography (CT) can predict their strength. We divided one supraspinatus tendon of 40 adult female white New Zealand rabbits and, after resection of the enthesis, sutured the tendon into a bony trough. Ten rabbits were killed immediately and 10 each at 1, 2, and 6 weeks postoperatively. The explanted tendons of both shoulders were imaged on CT and tested to failure. Compared to normal tendons (mean 210 +/- 42 N), the mean strength was very low at 0 weeks (57 +/- 21 N) and 1 week (86 +/- 33 N) (both p < 0.05); it had recovered by 6 weeks (324 +/- 66 N). Early on, suture pullout was the most common mode of failure, whereas at 6 weeks, mid-substance tears predominated (p < 0.05). Hypoattenuation on CT was associated with increased strength of the tendon-bone construct (p < 0.05). The strength of the surgical construct is very low in the early postoperative period. Therefore, the shoulder should be submitted only to loads not interfering with healing.

Restoration of strength despite low stress and abnormal imaging after Achilles injury.

PURPOSE: To determine the usefulness of clinical imaging in predicting the mechanical properties of rabbit Achilles tendons after acute injury. METHODS: We created a 2 x 7-mm full-thickness central tendon defect in one Achilles tendon of healthy rabbits. Rabbits in groups of 10 were killed immediately and 4 and 8 wk after surgery (n = 30). We then performed magnetic resonance (MR) imaging, ultrasonography (US), bone mineral densitometry (BMD), and mechanical testing to failure using a dual-cryofixation assembly on experimental and contralateral tendons. The main outcome measures included tendon dimensions, optical density (OD) of T1-weighted, proton density (PD), and T2-weighted MR sequences, US focal abnormalities, BMD of the calcaneus, and stress and peak load to failure. RESULTS: On MR imaging and US, all dimensions of the injured tendons after 2 wk and more were greater than those of the contralateral tendons (P < 0.05). The mean T1-weighted OD was greater at 4 wk (256 +/- 53) and 8 wk (184 +/- 24) than immediately after surgery (149 +/- 15). Mechanical stress was markedly lower in the experimental than in the contralateral tendons at both 4 wk (39 +/- 9 vs 77 +/- 16 N x mm(-2)) and 8 wk (58 +/- 6 vs 94 +/- 26 N x mm(-2); P < 0.05). Mean peak load to failure was significantly lower immediately after surgery (332 +/- 128 N) than at 4 and 8 wk (712 +/- 106 and 836 +/- 90 N, respectively). Both high T1-weighted OD (r = -0.73) and PD OD (r = -0.69) correlated with lower mechanical stress (P < 0.05). In the experimental tendons, higher T1-weighted OD correlated with lower peak load (r = -0.46; P < 0.05). CONCLUSIONS: Normal peak loads 4 wk after injury were withstood by an enlarged tendon of lower stress. These findings support progressive physical loading 4 wk after an Achilles tendon injury. T1-weighted OD constituted a marker of tendon mechanical recovery.