Effect of Active and Passive Function of the Posterosuperior Rotator Cuff on Compensatory Muscle Loads in the Shoulder.

Background: Irreparable posterosuperior rotator cuff (PSRC) tears have been shown to result in shoulder pain and loss of function. Purpose/Hypothesis: The purpose of this study was to determine the effect of the loss of active or passive function of the PSRC on compensatory muscle loads in the deltoid and the remaining rotator cuff. It was hypothesized that both deactivation and resection of the PSRC would result in load increases in the remaining muscles and that resection of the PSRC would result in even higher required compensatory forces than would mere deactivation. Study Design: Controlled laboratory study. Methods: A total of 7 cadaveric shoulders were tested using a biomechanical shoulder simulator with 10 independently controlled actuators for various muscles (anterior, middle, and posterior deltoid; inferior and superior subscapularis; latissimus dorsi; pectoralis major; teres minor; supraspinatus; and infraspinatus) and 3-dimensional motion tracking. The muscle loads representing the latissimus dorsi and pectoralis major were each held constant, and the remaining muscle actuator forces required to abduct the arm in the scapular plane were determined. The actuator forces corresponding with arm elevation from 20° to 65° were compared at 5° increments for 3 testing conditions: (1) intact, active PSRC; (2) intact, deactivated PSRC; and (3) resected PSRC and shoulder capsule. Results: In both the deactivated and resected states, the teres minor showed a significant increase in required muscle forces through nearly the entire tested range of arm elevation compared to the active state. This was also the case for the subscapularis but only at higher elevation angles. The deltoid demonstrated increased muscle forces of at least 1 of its subunits between 25° and 55° of elevation when comparing the deactivated state or resected state to the active state. However, through nearly the entire range of elevation, no statistically significant differences were found between the deactivated and resected states for any of the actuator loads representing muscle forces. Conclusion: The loss of active function of the PSRC led to compensatory loads on the remaining rotator cuff and deltoid, regardless of the passive presence of the PSRC as a supposed subacromial spacer. Clinical Relevance: The findings encourage the exploration of treatment procedures that mimic the active function of the PSRC when the rotator cuff itself is irreparable.

Development and Multisite Assessment of a Novel Shoulder Motion Joint Simulator.

Multiple biomechanical shoulder simulators have been described in the literature, with a trend toward increasing complexity to better simulate clinical scenarios. Our objective was to develop an advanced, novel shoulder joint simulator and compare outcomes at two separate institutions, for a typical shoulder joint motion simulation. Identical shoulder simulators were developed & deployed at both institutions. Eight cadaveric upper extremities were tested by simulating actively controlled, arm elevation in the plane of the scapula for two sequential test conditions (intact and nondestructive simulated cuff-tear), each repeated for a total of five trials. Muscle forces and joint translations were recorded for both conditions. The intact condition was repeated following simulated cuff-tear to assess effect of testing order. Statistical analyses were aimed at assessing repeatability and reproducibility of results within specimens, between specimens, and between institutions. The highest average forces were observed for the middle deltoid (233N or 32.5% body weight (BW)), followed by infraspinatus (99.0N), and posterior deltoid (93.7N) muscles. Differentiation between test conditions was unhindered by variability between repeated trials. Data from testing repeated over time, and between the two institutions were not significantly different. The novel shoulder simulator produced repeatable results with low trial-to-trial variation and outcomes were comparable between the two institutions. The results demonstrated a consistent response in muscle forces and humeral translation for the simulated rotator cuff tear condition. Such advanced shoulder simulators could thus be used for evaluating and optimizing surgical interventions and implant strategies.

Scapular motion in the presence of rotator cuff tears: a systematic review.

BACKGROUND: Rotator cuff tears (RCTs) remain a significant source of pain and disability in the shoulder. Although much work has been done in the study of the effects of rotator cuff tears on glenohumeral joint motion, much less has been done in understanding the effect of rotator cuff tearing on scapular motion or activation. It remains unknown whether scapular dyskinesis is causative or adaptive. The purpose of this study was to systematically review the literature to determine the relationship between rotator cuff tear presence and size on scapular motion, and if rotator cuff repair restored normal motion. METHODS: A systematic review using PRISMA guidelines was accomplished to include all studies with biomechanical or clinical outcomes of scapular motion in the presence of RCTs. Studies were excluded if they involved shoulder arthroplasty, rotator cuff tendinopathy, or shoulder impingement without an RCT. From 530 initial references, 42 manuscripts were selected for full review and cross referenced. All studies were evaluated for inclusion and exclusion criteria. RESULTS: Sixteen studies including 335 rotator cuff tears were included in the final review. There were several findings of interest in the literature. First, although all studies demonstrated scapular dyskinesis, they did not report a consistent pattern of motion in the presence of an RCT. In general, scapular posterior tilt was decreased, and scapular upward rotation was increased, especially in large tears, but the literature was unclear as to whether this was a result of the RCT or an adaptive attempt to maintain elevation. Larger RCTs resulted in more pronounced scapular dysfunction, but there was significant variability within studies. Further, dyskinesis was confounded by pain with more abnormal movement in symptomatic vs. asymptomatic RCTs, the latter of which were not different from normal healthy controls. Four studies addressed the effect of RCT on scapular mechanics and found that repair consistently improved it compared to the normal side, but the time to normalization varied between 5 months and 2 years. CONCLUSION: Scapular motion is abnormal in the presence of an RCT, but the literature is inconsistent regarding a universally affected variable or consistent degree of scapular dysfunction in this setting. Furthermore, it remains unknown which changes are adaptive vs. pathologic. Understanding the relationship between rotator cuff tearing and scapular dyskinesis will require better biomechanical models that consider scapular dyskinesis in their design.

The Biomechanical Performance of the Latest All-Inside Meniscal Repair Devices.

PURPOSE: To evaluate the biomechanical characteristics of recently introduced meniscal repair devices with a hand-tied, inside-out meniscal suture in a human meniscus model. METHODS: In detached adult human menisci, vertical longitudinal cuts were created 3 mm from the synovial-meniscal junction, simulating a bucket-handle meniscal tear. Each cut was repaired using a single device. Group 1 received a vertical mattress suture of No. 2-0 OrthoCord; group 2, TrueSpan device with PEEK (polyether ether ketone) anchors containing No. 2-0 OrthoCord suture; group 3, TrueSpan device with biodegradable poly-lactide-co-glycolide (PLGA) anchors containing No. 2-0 OrthoCord suture; group 4, Meniscal Cinch II device; group 5, AIR meniscal repair device; and group 6, FasT-Fix 360 device. All samples were preloaded at 5 N and cycled 200 times between 5 and 20 N. The specimens that survived cyclic loading were destructively tested at 12.5 mm/s. Endpoints included maximum load, displacement, stiffness, and failure mode. RESULTS: The mean failure loads were as follows: 95.8 N for OrthoCord suture, 87.1 N for TrueSpan with PEEK, 84.6 N for TrueSpan with PLGA, 48.6 N for Meniscal Cinch II, 72.3 N for AIR, and 68.1 N for FasT-Fix 360. Repairs performed with OrthoCord suture (P = .002) and both TrueSpan devices (P < .03) but not the FasT-Fix 360 device or AIR device were statistically significantly stronger than Meniscal Cinch II repairs. Mean cyclic displacement measured 1.1 mm for OrthoCord, 1.5 mm for TrueSpan with PEEK, 1.5 mm for TrueSpan with PLGA, 2.1 mm for Meniscal Cinch II, 1.1 mm for AIR, and 1.4 mm for FasT-Fix 360. The Meniscal Cinch II device showed more displacement than all other devices (P < .05). The FasT-Fix 360, AIR, and Meniscal Cinch II devices failed by anchor pullout from the peripheral meniscus. OrthoCord and both TrueSpan devices failed by suture pulling through the bucket-handle tissue. CONCLUSIONS: OrthoCord suture is stronger than the AIR, FasT-Fix 360, and Meniscal Cinch II devices. The TrueSpan device with PEEK and TrueSpan device with PLGA are stronger than the Meniscal Cinch II device. The Meniscal Cinch II device failed during cyclic loading with greater cyclic displacement than the AIR device, FasT-Fix 360 device, OrthoCord, and TrueSpan device with PEEK. The Meniscal Cinch II, AIR, and FasT-Fix 360 devices failed by anchor pullout, whereas OrthoCord and both TrueSpan devices failed by suture pull-through. CLINICAL RELEVANCE: Some newly introduced all-inside meniscal repair devices show inferior failure strength compared with earlier versions that might adversely impact clinical outcomes.

A Biomechanical Comparison of Six Suture Configurations for Soft Tissue-Based Graft Traction and Fixation.

PURPOSE: The purpose of this study was to compare 6 different graft fixation techniques to determine the preparation speed, fixation security, biomechanical strength, and resultant tissue trauma. METHODS: Six different techniques (10 samples each): #2 OrthoCord Krackow stitch, #2 FiberWire Krackow stitch, SpeedTrap, WhipKnot, Loop-in-loop stitch were created in the distal 3 cm of 9 cm bovine flexor tendons. The proximal 3 cm tendon segment was clamped in a testing machine and the distal suture ends secured by pneumatic grips. 3 preload cycles (10N-100N) and 50N static load was followed by 500 cycles (50N-200N) and then loaded to failure. Graft preparation times, 100 and 500 cycle displacement, maximum failure load, stiffness, and failure mode were recorded. RESULTS: Representative graft preparation times were: OrthoCord Krackow (247s), FiberWire Krackow (401s), FiberLoop (177s), SpeedTrap (42s), WhipKnot (39s), Loop-in-loop (45s). No WhipKnots survived cyclic loading. 100 cycle displacements were: OrthoCord Krackow (11.5 ± 3.9 mm), FiberWire Krackow (8.9 ± 1.2 mm), FiberLoop (14.2 ± 6.1 mm), SpeedTrap (8.8 ± 2.5 mm), Loop-in-loop (10.4 ± 2.9 mm). FiberLoop displaced significantly more than all others (P = .016). Maximum failure loads were: OrthoCord Krackow (364 ± 24N), FiberWire Krackow (375 ± 45N), FiberLoop (413 ± 95N), SpeedTrap (437 ± 65N), WhipKnot (153 ± 42N), Loop-in-loop (329 ± 112N). The most common failure mode was suture breaking. FiberWire containing constructs (Krackow and FiberLoop) shredded or cut through ("cheese wiring") prior to failure in a majority. CONCLUSIONS: SpeedTrap, WhipKnot and Loop-in-loop were quickest to create (under 1 minute). The Krackow, SpeedTrap, WhipKnot, and Loop-in-Loop did not damage the tendon during cyclic loading. SpeedTrap and Krackow had the least displacement. FiberLoop displaced more than all other groups (P = .016). No WhipKnot completed cyclic loading. The SpeedTrap (437N) and FiberLoop (413N) had the highest ultimate strength. CLINICAL RELEVANCE: While the SpeedTrap and FiberLoop are the strongest techniques, the FiberLoop shreds the tendon, displaced the most, and took longer to create. Based on these results, the SpeedTrap demonstrates the best overall performance.

Suture-Based Debris Behavior in the Draining Lymph Nodes of a Porcine Knee: A Study of Silicone, Polyethylene and Carbon.

PURPOSE: To determine whether debris from a silicone core suture has an observable intra-articular or extra-articular impact or can be shown to migrate into the lymphatic system. METHODS: Using a porcine stifle joint model, 2 study groups were created: 1 group used silicone-suture particles created by rupturing hand-tied knots of a nonabsorbable suture with an outer sheath of ultrahigh molecular-weight polyethylene, an inner polyester sheath and a medical-grade silicone/sodium chloride-filled core. The second group used a mixture of 3 vitreous carbon particles sizes. Twelve Yorkshire pigs were randomly assigned to each group. Only 1 type of study particle was used in a single animal to avoid cross-contamination. Half of the study materials were placed by arthrotomy into the joint and, after capsular closure, the remaining half were placed on the joint capsule before skin closure. Six weeks postimplantation, the stifle joints and regional lymph nodes were examined macroscopically and microscopically. The extracapsular and intra-articular tissue and the lymph nodes were examined. RESULTS: Macroscopically, none of the 12 silicone-suture particle group limbs had abnormal macroscopic observations. Of the 12 limbs in the carbon particle group, 8 demonstrated discoloration in the extracapsular, intra-articular and lymph node tissues; 6 limbs demonstrated joint discoloration (extracapsular and intra-articular). The silicone-suture particle group showed lymph node inflammation in 25% of the iliac lymph nodes and 42% of the inguinal lymph nodes but none in the popliteal lymph nodes. In the carbon particle group, 100% of the iliac lymph nodes, 75% of the inguinal lymph nodes and 8% of the popliteal nodes showed inflammation. No silicone-suture debris migration was observed microscopically. Visible carbon particle migration was present in 100% of the iliac lymph nodes and in 50% of the inguinal lymph nodes but in none (0) of the popliteal lymph nodes. The carbon particle group also showed inflammation in these areas. CONCLUSIONS: Carbon particles in the porcine knee migrated into the lymph nodes (iliac 100%; inguinal 50%, popliteal 0); gross discoloration was observed in 8 of 12 specimens. Silicone-suture particles could not be found in the lymph nodes, and no macroscopic joint damage was observed. CLINICAL RELEVANCE: The clinical use of a suture containing a silicone/salt central core is common, and the porcine lymphatic model suggests that this type of suture material does not result in clinically significant silicone particulates.

Safety evaluation of a laxity-minimizing suture at 5 days and 6 weeks after repair of a sheep infraspinatus tendon.

BACKGROUND: The ideal rotator cuff repair achieves high initial fixation strength and secure tendon-to-bone apposition until biological healing occurs. A suture that reacts to the local stress environment by minimizing suture laxity across the repair could theoretically maintain soft-tissue apposition to bone and therefore improve healing. METHODS: By use of an in vivo ovine shoulder model, the infraspinatus tendon was transected and then repaired with either a laxity-minimizing suture or a traditional high tensile suture. The purpose of this study was to evaluate both sutures' safety at 5 days and 6 weeks after repair. RESULTS: The macroscopic and microscopic analyses of the repair sites showed similar amounts of surgical trauma. There was no evidence of cheese wiring or tissue necrosis of the repaired tendons for either suture. There was no evidence of systematic toxicity in any animal. The maximum gap between cut edges of the tendon for repairs with the predicate suture was approximately twice the gap for the laxity-minimizing suture. CONCLUSION: The laxity-minimizing suture was as safe at 5 days and 6 weeks as the predicate suture. Neither suture contributed to local tissue damage or particle generation leading to adverse systematic consequences. An additional observation was that the maximum gap between cut edges of the tendon for repairs with the predicate suture was approximately twice the gap for the laxity-minimizing suture.

Development of a Porcine Lymph Drainage Model to Study Particle Migration.

Background: Intra-articular wear particulate migration from the knee joint has been studied in various animal models as well as postmortem in patients who received total knee joint replacement. However, there still exists a need for a simple, yet analogous animal model for tracking the migration of wear debris from the knee joint, especially through the draining lymph nodes. Methods and Results: To fill this need, a proof-of-concept porcine model was developed for particle migration from the knee joint into the surrounding lymphatic system. Vitreous carbon particles were deposited both intra-articularly and extracapsularly in a bilateral manner to the hind limbs in pigs (n = 6). The regional/draining lymph nodes were qualitatively assessed weekly by a veterinarian by manual palpation to detect any enlargement or change in consistency when compared to the initial assessment before the surgical procedure. At 6 weeks, the draining lymph nodes were harvested and processed for histology. Microscopic evaluation showed carbon particle migration from the knee into 100% of the iliac lymph nodes, 50% of the inguinal lymph nodes, and 0% of the popliteal lymph nodes. Discussion: Overall, this study established a needed animal model for evaluating carbon particle migration to the draining lymph nodes from the knee joint.

A comparison of three adjustable cortical button ACL fixation devices.

PURPOSE: Adjustable cortical fixation has gained popularity recently for ACL reconstruction; however, one concern with these types of devices is the potential for laxity occurring post-operatively. An indicator of clinical laxity is cyclic displacement during bench-top mechanical testing. The hypothesis was that the cyclic displacement and maximum strength of different adjustable buttons currently on the market would vary depending on their mechanism of fixation. METHODS: Three devices were studied: Biomet's ToggleLoc with ZipLoop Technology (ZL), Arthrex's TightRope RT (TR), and DePuy Mitek's RIGIDLOOP Adjustable (RLA). Each was tested in isolation on a servohydraulic test machine. The implants were pre-conditioned from 5 to 67 N for 10 cycles and then tested from 50 to 250 N for 1000 cycles at 1 Hz. Following cyclic loading, a load to failure test was conducted at a rate of 20 mm/min. Total displacement was calculated from the 1st to the 1000th cycle, and ultimate strength and failure mode were recorded. RESULTS: Results showed that the RLA had significantly lower displacement versus both the TR (p = 0.012) and the ZL (p < 0.001). The TR also showed significantly lower displacement than the ZL (p < 0.001). The RLA and the ZL were both significantly stronger than the TR (p < 0.001). CONCLUSIONS: Both the RLA and TR showed clinically acceptable amounts of cyclic displacement and maximum strength. The clinical relevance of this study is that cyclic displacement results from bench-top tests may correlate with post-operative laxity, which ideally should be kept to a minimum.

Biocomposite Implants Composed of Poly(Lactide-co-Glycolide)/ß-Tricalcium Phosphate: Systematic Review of Imaging, Complication, and Performance Outcomes.

PURPOSE: To determine the degradation and performance outcomes of poly(lactide-co-glycolide)/ß-tricalcium phosphate (PLGA/ß-TCP) implants. METHODS: A MEDLINE and Embase search for randomized or nonrandomized controlled studies and prospective or retrospective case series that used biocomposite interference screws or suture anchors composed of PLGA/ß-TCP was performed. Main outcomes included volume of implant resorption and incidence of osteoconductivity at implant sites, imaging findings, adverse events, and the frequency of reoperations. RESULTS: A total of 13 studies representing 668 patients with either knee or shoulder implants were included. Median follow-up was 28 months (range: 12-37 months). Biocomposite implants lost 88% of their original volume during follow-up. Osteoconductivity at the implant site was identified in 63% of cases. Adverse events included tunnel widening (3%), effusion (5%), and cyst formation (4%). Synovitis was not reported. CONCLUSIONS: Biocomposite interference screws or suture anchors composed of PLGA/ß-TCP almost fully absorb over 3 years while promoting osteoconductivity with few reported adverse events. LEVEL OF EVIDENCE: Level IV, systematic review of Level I-IV studies.

The "Lever Sign": a new clinical test for the diagnosis of anterior cruciate ligament rupture.

PURPOSE: A new clinical test for the diagnosis of ACL rupture is described: the so-called "Lever Sign". This prospective study on four groups of patients divided subjects on the basis of MRI findings (complete or partial ACL lesion) and the clinical phase of the injury (acute or chronic). The hypothesis was that this manual test would be diagnostic for both partial and complete tears of the ACL regardless of the elapsed time from injury. METHODS: A total of 400 patients were evaluated and divided into four, equal-sized groups based on time elapsed from injury and MRI findings: Group A (acute phase with positive MRI for complete ACL rupture), Group B (chronic phase with positive MRI for complete ACL rupture), Group C (acute phase with positive MRI for partial ACL rupture), and Group D (chronic phase with positive MRI for partial ACL rupture). Clinical assessment was performed with the Lachman test, the Anterior Drawer test, the Pivot Shift test, and the Lever Sign test. The Lever Sign test involves placing a fulcrum under the supine patient's calf and applying a downward force to the quadriceps. Depending on whether the ACL is intact or not, the patient's heel will either rise off of the examination table or remain down. Additionally, the Lever Sign test was performed on the un-injured leg of all 400 patients as a control. RESULTS: All tests were nearly 100 % sensitive for patients with chronic, complete tears of the ACL. However, for patients with acute, partial tears, the sensitivity was much lower for the Lachman test (0.42), Anterior Drawer test (0.29), and Pivot Shift test (0.11), but not the Lever Sign test (1.00). CONCLUSION: In general, chronic, complete tears were most successfully diagnosed but acute, partial tears were least successfully diagnosed. The Lever Sign test is more sensitive to correctly diagnosing both acute and partial tears of the ACL compared with other common manual tests. The clinical relevance is that some ACL ruptures may be more accurately diagnosed.

Cyclic biomechanical testing of biocomposite lateral row knotless anchors in a human cadaveric model.

PURPOSE: The purpose of this study was to assess the mechanical performance of biocomposite knotless lateral row anchors based on both anchor design and the direction of pull. METHODS: Two lateral row greater tuberosity insertion sites (anterior and posterior) were identified in matched pairs of fresh-frozen human cadaveric shoulders DEXA (dual energy X-ray absorptiometry) scanned to verify comparability. The humeri were stripped of all soft tissue and 3 different biocomposite knotless lateral row anchors: HEALIX Knotless BR (DePuy Mitek, Raynham MA), BioComposite PushLock (Arthrex, Naples, FL), and Bio-SwiveLock (Arthrex). Fifty-two anchors were distributed among the insertion locations and tested them with either an anatomic or axial pull. A fixed-gauge loop (15 mm) of 2 high-strength sutures from each anchor was created. After a 10-Nm preload, anchors were cycled from 10 to 45 Nm at 0.5 Hz for 200 cycles and tested to failure at 4.23 mm/second. The load to reach 3 mm and 5 mm displacement, ultimate failure load, displacement at ultimate failure, and failure mode were recorded. RESULTS: Threaded anchors (Bio-SwiveLock, P = .03; HEALIX Knotless, P = .014) showed less displacement with anatomic testing than did the nonthreaded anchor (BioComposite PushLock), and the HEALIX Knotless showed less overall displacement than did the other 2 anchors. The Bio-SwiveLock exhibited greater failure loads than did the other 2 anchors (P < .05). Comparison of axial and anatomic loading showed no maximum load differences for all anchors as a whole (P = .1084). Yet, anatomic pulling produced higher failure loads than did axial pulling for the Bio-SwiveLock but not for the BioComposite PushLock or the HEALIX Knotless. The nonthreaded anchor (BioComposite PushLock) displayed lower failure loads than did both threaded anchors with axial pulling. CONCLUSIONS: Threaded biocomposite anchors (HEALIX Knotless BR and Bio-SwiveLock) show less anatomic loading displacement and higher axial failure loads than do the nonthreaded (BioComposite PushLock) anchor. The HEALIX Knotless BR anchor showed less displacement than did the BioComposite PushLock and Bio-SwiveLock anchors. Neither axial nor anatomic loading had an effect on overall anchor displacement. CLINICAL RELEVANCE: Because of the strength profiles exhibited, this study supports the use of biocomposite anchors, which have definite advantages over polyetheretherketone (PEEK) and metal products. However, the nonthreaded BioComposite PushLock anchor cannot be recommended.

Kinematic effects of a pedicle-lengthening osteotomy for the treatment of lumbar spinal stenosis.

OBJECT: Lumbar spinal stenosis (LSS) may lead to disabling neurogenic symptoms and has traditionally been treated using open laminectomy. A new technique for correcting LSS involves lengthening the lumbar pedicles through bilateral percutaneous pedicle osteotomies. In this paper, the authors' goal was to evaluate the changes in spinal canal dimensions and kinematic behavior after pedicle-lengthening osteotomies. METHODS: The kinematic behavior of 8 cadaveric lumbar segments was evaluated intact and after bilateral pedicle-lengthening osteotomies at the L-4, L-5, and L-4 and L-5 levels. Testing was conducted with and without a compressive preload using a custom kinematic apparatus that allowed for 3D tracking of each vertebra during flexion-extension, right-left bending, and right-left rotation. A validated finite element (FE) spine model was used to measure the changes in the cross-sectional area of the spinal canal and neural foramen after 2-, 3-, and 4.5-mm simulated pedicle-lengthening osteotomy procedures. RESULTS: The overall and segmental kinematics were not significantly altered after the pedicle-lengthening osteotomy procedure at the L-4 and/or L-5 pedicles. The kinematic signatures of the intact and lengthened states were similar for all motion pairs. The FE spine model yielded kinematics predictions within or close to the 95% confidence interval for the cadaveric data. The FE spine demonstrated substantial, pedicle length-dependent enlargement of the cross-sectional areas of the spinal canal and neural foramen after simulated pedicle lengthening. CONCLUSIONS: Bilateral pedicle-lengthening osteotomies produced substantial increases in the cross-sectional areas of the spinal canal and neural foramen without significantly altering normal spinal kinematics. This technique deserves further study as a less invasive treatment option for LSS.

Biomechanical comparison of single-row arthroscopic rotator cuff repair technique versus transosseous repair technique.

This study determined the effect of tear size on gap formation of single-row simple-suture arthroscopic rotator cuff repair (ARCR) vs transosseous Mason-Allen suture open RCR (ORCR) in 13 pairs of human cadaveric shoulders. A massive tear was created in 6 pairs and a large tear in 7. Repairs were cyclically tested in low-load and high-load conditions, with no significant difference in gap formation. Under low-load, gapping was greater in massive tears. Under high-load, there was a trend toward increased gap with ARCR for large tears. All repairs of massive tears failed in high-load. Gapping was greater posteriorly in massive tears for both techniques. Gap formation of a modeled RCR depends upon the tear size. ARCR of larger tears may have higher failure rates than ORCR, and the posterior aspect appears to be the site of maximum gapping. Specific attention should be directed toward maximizing initial fixation of larger rotator cuff tears, especially at the posterior aspect.

Mechanical properties of chest protectors and the likelihood of ventricular fibrillation due to commotio cordis.

Sudden death resulting from ventricular fibrillation (VF) caused by a nonpenetrating chest wall impact, known as commotio cordis (CC), is the second leading cause of death among young athletes. To date, seven young athletes wearing chest protectors have died from CC. The purpose of this study was to determine whether a relationship exists between mechanical properties of chest protectors and occurrence of VF, previously determined by Weinstock et al., using an established swine model. A servo-hydraulic material tester was used to determine properties of the chest protectors, including displacement, permanent deformation, stiffness, and area of pressure distribution. These properties were then compared with the occurrence of VF. We found that a decreased proportion of hits resulting in VF was significantly associated (R2 = 0.59, p = 0.001) with an increase in the area of pressure distribution. These findings are a limited, but crucial, first step in understanding the prevention of this complex and perplexing phenomenon.

Initial fixation strength of massive rotator cuff tears: in vitro comparison of single-row suture anchor and transosseous tunnel constructs.

PURPOSE: The purpose of this study was to compare the in vitro repair integrity of massive rotator cuff tears fixed with transosseous tunnel and single-lateral row suture anchor techniques. METHODS: A 5 x 2-cm crescent-shaped rotator cuff tear was created in 6 matched pairs of cadaveric shoulders. Paired shoulders were repaired with 3 transosseous tunnels and 6 Mason-Allen sutures or with 3 screw-in suture anchors and 6 simple sutures. The repairs were cyclically loaded at physiologic forces along the respective directions of pull when the arm was in 90 degrees of scapular plane elevation. Gap formation and repair displacements were monitored with digital video imaging at 3 sites for each repair. RESULTS: There was no significant difference between the maximal gapping of the repair constructs. After 4,000 cycles, the mean maximal gapping at any position along the repair was 6.2 +/- 2.99 mm in the transosseous tunnel construct and 4.9 +/- 1.27 mm in the suture anchor repair construct (P = .40). Gapping was significantly less in the anterior region when compared with the posterior region of the repair (P = .015). CONCLUSIONS: There is no difference in cyclic loading of transosseous and single-row suture anchor repair techniques. Significantly greater gap formation occurs at the posterior aspect of repairs of massive rotator cuff tears in this in vitro model. CLINICAL RELEVANCE: Initial fixation strength of single-row suture anchor repairs is equivalent to that of transosseous repairs. Further research is required to determine the unknown clinical significance of increased posterior repair gap formation.

The dynamic flexion/extension properties of the lumbar spine in vitro using a novel pendulum system.

The biomechanical properties of the ligamentous cadaver spine have been previously examined using a variety of experimental testing protocols. Ongoing technical challenges in the biomechanical testing of the spine include the application of physiologic compressive loads and the application of dynamic bending moments while allowing unconstrained three-dimensional motion. The purpose of this study was to report the development of a novel pendulum apparatus that addressed these challenges and to determine the effects of various axial compressive loads on the dynamic biomechanical properties of the lumbar functional spinal unit (FSU). Lumbar FSUs were tested in flexion and extension under five axial compressive loads chosen to represent physiologic loading conditions. After an initial rotation, the FSUs behaved as a dynamic, underdamped vibrating elastic system. Bending stiffness and coefficient of damping increased significantly as the compressive pendulum load increased. The apparatus described herein is a relatively simple approach to determining the dynamic bending properties of the FSU, and potentially disc arthroplasty devices. It is capable of applying physiologic compressive loads at dynamic rates without constraining the kinematics of the joints, crucial requirements for testing FSUs in vitro.

Accuracy of circular contact area measurements with thin-film pressure sensors.

Contact area is often used to characterize the biomechanical properties of joints, especially in testing of injury and joint replacement. Several methods have been developed to measure contact area, including piezo-resistive thin-film arrays. The purpose of this study was to determine the accuracy with which one of these systems (Tekscan, Inc., South Boston, MA) could measure the contact area of flat-ended circular indenters of varying known sizes. Static loads ranging from 1000 to 7000 N were applied to four flat, circular indenters (1140, 2027, 3167, and 4560 mm(2)) and the contact areas were recorded with Tekscan 5076 sensor. Similar testing was carried out on a 4000 sensor. I-scan software (Tekscan Inc., South Boston, MA) was used to analyze the Tekscan-recorded area measurements. The Tekscan data were also post-processed to filter out sensel signal intensity values that were at least two standard deviations from the average sensel signal intensity values of the sensor matrix. Unprocessed Tekscan measurements with the 5076 sensor had area percent errors ranging from 5% to 27%. The filtering algorithm reduced most errors to less than 1%. Similar trends of improved accuracy with post-filtering were found with the 4000 sensor. While this method of thresholding out the sensels with the lowest signal intensity values may not work for all surfaces and indenter shapes, it provides a new approach to improve the accuracy of contact area measurements collected with the Tekscan system.

High rate of fusion in sheep cervical spines following anterior interbody surgery with absorbable and nonabsorbable implant devices.

STUDY DESIGN: Fourteen sheep were fused using anterior interbody implants at C2-C3 and C4-C5 and followed for 6 months. OBJECTIVE: To evaluate the effect of absorbable and nonabsorbable implants on fusion rate, cage migration, and implant integrity. SUMMARY OF BACKGROUND DATA: Despite the high clinical success rate with metallic plates and interbody grafting, complications such as dysphagia, imaging artifacts, and revision difficulties exist. Less permanent and lower profile implants could minimize these problems. METHODS: Four treatments were studied: 1) carbon fiber-reinforced polymer (CFRP) cage alone, 2) CFRP cage with an absorbable tension band, 3) absorbable cage with an absorbable tension band, and 4) absorbable cage with a titanium plate. Fusion was assessed using radiographs, biomechanical testing, and micro-CT analysis. RESULTS: Treatments with the CFRP cage alone had the lowest fusion rate at 3 months (2/6) and 6 months (4/6). The CFRP cage with the absorbable strap treatments had 5/6 fusions at 6 months. The absorbable cage with absorbable strap also had 5/6 fusions, but two of the cages fractured. The absorbable cage with titanium plate had 5/6 fusions, but the one partial fusion was attributed to poor screw and plate placement. CONCLUSION: Using both absorbable and nonabsorbable implants, high fusion rates were achieved in the challenging sheep cervical spine model. However, the absorbable cages were not able to withstand the mechanical forces during the 6-month survival period.