Update on musculoskeletal applications of magnetic resonance-guided focused ultrasound.

Magnetic resonance-guided focused ultrasound (MRgFUS) is a noninvasive, incisionless, radiation-free technology used to ablate tissue deep within the body. This technique has gained increased popularity following FDA approval for treatment of pain related to bone metastases and limited approval for treatment of osteoid osteoma. MRgFUS delivers superior visualization of soft tissue targets in unlimited imaging planes and precision in targeting and delivery of thermal dose which is all provided during real-time monitoring using MR thermometry. This paper provides an overview of the common musculoskeletal applications of MRgFUS along with updates on clinical outcomes and discussion of future applications.

Ultrasound-guided bursal injections.

The native bursa is a structure lined by synovium located adjacent to a joint which may serve to decrease friction between the tendons and overlying bone or skin. This extra-articular structure can become inflamed resulting in bursitis. Steroid injections have proven to be an effective method of treating bursal pathology in various anatomic locations. Performing these procedures requires a thorough understanding of relevant anatomy, proper technique, and expected outcomes. Ultrasound is a useful tool for pre procedure diagnostic evaluation and optimizing needle position during these procedures while avoiding adjacent structures. The purpose of this article is to review core principles of ultrasound-guided musculoskeletal procedures involving bursae throughout the upper and lower extremities.

Ultrasound-guided joint interventions of the lower extremity.

The purpose of this article is to better understand the role ultrasound plays in lower extremity joint interventions. Ultrasound is an important and reliable tool diagnostically and therapeutically. Real-time feedback, lack of ionizing radiation, and dynamic maneuverability make ultrasound an important tool in the proceduralist's armament. This article will touch upon the important anatomic considerations, clinical indications, and technical step-by-step details for lower extremity ultrasound interventions. Specifically, we will look at interventions involving the hip, knee, ankle, and foot. In addition, this article will discuss the roles corticosteroid and platelet-rich plasma may play in certain interventions.

Botryomycosis: a rare mimic of sarcoma as an initial presentation of acquired immunodeficiency syndrome.

Botryomycosis is a rare granulomatous response to chronic bacterial infection most frequently associated with Staphylococcus aureus. This disease, which predominantly affects immunocompromised patients, may present with cutaneous, visceral, or soft tissue manifestations. Soft tissue involvement typically has an aggressive mass-like appearance on imaging which can be concerning for malignancy. In immunocompromised patients, botryomycosis can resemble fungal infection both clinically and histologically; therefore, definitive diagnosis requires tissue sampling along with histological and microbiological analysis. Presented here is a 25-year-old man with an enlarging intramuscular soft tissue mass of the right forearm as his first presentation of undiagnosed acquired immunodeficiency syndrome (AIDS). MR imaging showed a mildly T2 hyperintense and enhancing mass with infiltrative margins extending through tissue planes. Biopsy of the mass revealed Staphylococcus aureus-associated botryomycosis, which improved with nonsurgical treatment employing antibiotics. Unfortunately, the patient subsequently expired from other manifestations of his new AIDS diagnosis. This case describes the MR and PET-CT appearance of botryomycosis and also underscores that infection can mimic sarcoma, particularly in the setting of immunodeficiency.

Musculoskeletal Applications of Magnetic Resonance-Guided Focused Ultrasound.

Magnetic resonance-guided focused ultrasound (MRgFUS) is a novel noninvasive therapy that uses focused sound energy to thermally ablate focal pathology within the body. In the United States, MRgFUS is approved by the Food and Drug Administration for the treatment of uterine fibroids, palliation of painful bone metastases, and thalamotomy for the treatment of essential tremor. However, it has also demonstrated utility for the treatment of a wide range of additional musculoskeletal (MSK) conditions that currently are treated as off-label indications. Advantages of the technology include the lack of ionizing radiation, the completely noninvasive technique, and the precise targeting that offer unprecedented control of the delivery of the thermal dose, as well as real-time monitoring capability with MR thermometry. In this review, we describe the most common MSK applications of MRgFUS: palliation of bone metastases, treatment of osteoid osteomas, desmoid tumors, facet arthropathy, and other developing indications.

Magnetic resonance imaging evaluation of normal glenoid length and width: an anatomic study.

PURPOSE: The purpose of this study was to evaluate the measured dimensions of the normal glenoid on sagittal magnetic resonance (MR) imaging to determine whether a fixed ratio of glenoid length and width can be determined. METHODS: MR images of 90 glenoids in 84 patients were analyzed. The mean age was 54.8 years, with 44 male and 40 female patients. Glenoid length and width at the widest dimension were measured and recorded by 3 independent examiners. The ratio of length to width and the ratio of the length of the superior pole at the widest point to the total length were calculated. Intraclass correlation coefficients, Spearman and Pearson correlations, regression analysis with cross validation, and coefficients of variation were calculated. RESULTS: The mean glenoid length was 37.5 ± 3.8 mm, whereas the mean width was 24.4 ± 2.9 mm. The mean ratio of length to width was 1.55 ± 0.1, whereas the mean ratio of the distance from the superior pole to the widest point to the total glenoid length was 0.64 ± 0.03. The calculated ratios were less variable than the absolute length and width. Cross validation of length for width showed a 95% prediction band width of 4.48 mm, with an average absolute error of prediction of 1.46 mm, and was equally specific when separated by gender. The width was equal to 0.65 times the length. CONCLUSIONS: Measurement of glenoid length and width using MR imaging results in a consistent ratio of length to width independent of patient age and gender, where the width was equal to 0.65 times the length at a point two-thirds along the inferosuperior axis. LEVEL OF EVIDENCE: Level IV, case series.

Effect of interference screw depth on fixation strength in biceps tenodesis.

PURPOSE: The purpose of this study was to assess the biomechanical performance of the long head of the biceps tenodesis with an interference screw with respect to screw depth. METHODS: Twenty-one human cadaveric shoulders were randomized into 3 treatment groups (7 each): interference screw placed flush to the humeral cortex, 50% proud, or fully recessed. Bone density was determined, and subpectoral biceps tenodesis was performed with 8 × 12 mm Bio-Tenodesis screws (Arthrex, Naples, FL). Each construct was cyclically loaded from 5 to 70 N for 500 cycles at 1 Hz and then pulled to failure at 1 mm/s. Relative actuator displacement was calculated from cyclic testing. Maximum load, elongation, linear stiffness, and failure mode were recorded from pull-to-failure testing. Because of numerous failures during cyclic testing, the final load data from the fully recessed group were not statistically analyzed. The remaining groups were compared by use of a 2-tailed, Student unpaired t test and χ(2) analysis. RESULTS: There was no significant difference in displacement among groups during cyclic testing. Five specimens in the recessed group failed during cyclic testing, whereas 2 specimens and 0 specimens failed in the proud and flush groups, respectively. The maximum loads sustained were 281.6 ± 77.8 N, 184.5 ± 56.3 N, and 209.1 ± 57.0 N for the flush group, 50% proud group, and recessed group (in those specimens surviving cyclical loading), respectively. CONCLUSIONS: Placement of a Bio-Tenodesis screw flush to the humeral cortex is preferred for maximum fixation strength in subpectoral biceps tenodesis. A screw placed to 50% depth may be effective in the laboratory setting, but recessed placement is more variable and requires additional fixation. The fully recessed group resulted in 5 of 7 failures during cyclical loading, with no specimens failing in the flush group. CLINICAL RELEVANCE: This study shows the importance of determining the optimal depth of interference screw placement during biceps tenodesis to obtain optimal biomechanical performance and reduce the risk of fixation failure.

Clinical outcomes of reverse total shoulder arthroplasty in patients aged younger than 60 years.

BACKGROUND: Reverse total shoulder arthroplasty (RTSA) has been indicated primarily for patients aged older than 65 years with symptomatic rotator cuff deficiency, poor function, and pain. However, conditions that benefit from RTSA are not restricted to an elderly population. This study evaluates a consecutive series of RTSA patients aged younger than 60 years. METHODS: We evaluated 36 shoulders (mean age, 54 years) at a mean follow-up of 2.8 years (range, 24-48 months). Of these shoulders, 30 (83%) had previous surgery, averaging 2.5 procedures per patient. The preoperative conditions compelling RTSA were as follows: failed rotator cuff repair (12), fracture sequelae (11), failed arthroplasty (5), instability sequelae (4), cuff tear arthropathy (CTA) (4), and rheumatoid arthritis (2). Follow-up examinations included range-of-motion and strength testing, as well as Single Assessment Numeric Evaluation, visual analog scale, Simple Shoulder Test, American Shoulder and Elbow Surgeons (ASES), and Constant scores. Preoperative and postoperative radiographs were reviewed for component loosening and scapular notching. Failure criteria were defined as undergoing revision, having gross loosening, or having an ASES score below 50. RESULTS: The mean Single Assessment Numeric Evaluation score improved from 24.4 to 72.0; the visual analog scale pain score improved from 6 to 2.1. The Simple Shoulder Test score improved from 1.4 to 6.2, and the ASES score improved from 31.4 to 65.8. Active forward elevation improved from 56° to 121°. The normalized postoperative mean Constant score was 54.3. In 9 patients (25.0%), we recorded an ASES score below 50, and these cases were considered failures. CONCLUSION: RTSA can improve shoulder function in a younger, complex patient population with poor preoperative functional ability. This study's success rate was 75% at 2.8 years. This is a limited-goals procedure, and longer-term studies are required to determine whether similar results are maintained over time.

The high failure rate of biologic resurfacing of the glenoid in young patients with glenohumeral arthritis.

BACKGROUND: The current study evaluated the outcomes of biologic resurfacing of the glenoid using a lateral meniscus allograft or human acellular dermal tissue matrix at intermediate-term follow-up. METHODS: Forty-five patients (mean age, 42.2 years) underwent biologic resurfacing of the glenoid, and 41 were available for follow-up at a mean of 2.8 years. Lateral meniscal allograft resurfacing was used in 31 patients and human acellular dermal tissue matrix interposition in 10. Postoperative range of motion and clinical outcomes were assessed at the final follow-up. RESULTS: The overall clinical failure rate was 51.2%. The lateral meniscal allograft cohort had a failure rate of 45.2%, with a mean time to failure of 3.4 years. Human acellular dermal tissue matrix interposition had a failure rate of 70.0%, with a mean time to failure of 2.2 years. Overall, significant improvements were seen compared with baseline with respect to the visual analog pain score (3.0 vs. 6.3), American Shoulder and Elbow Surgeons score (62.0 vs. 36.8), and Simple Shoulder Test score (7.0 vs. 4.0). Significant improvements were seen for forward elevation (106° to 138°) and external rotation (31° to 51°). CONCLUSION: Despite significant improvements compared with baseline values, biologic resurfacing of the glenoid resulted in a high rate of clinical failure at intermediate follow-up. Our results suggest that biologic resurfacing of the glenoid may have a minimal and as yet undefined role in the management of glenohumeral arthritis in the young active patient over more traditional methods of hemiarthroplasty or total shoulder arthroplasty.

Role of the superior labrum after biceps tenodesis in glenohumeral stability.

BACKGROUND: Little is known about the role that a torn superior labrum (SLAP) plays in glenohumeral stability after biceps tenodesis. This biomechanical study evaluated the contribution of a type II SLAP lesion to glenohumeral translation in the presence of biceps tenodesis. The authors hypothesize that subsequent to biceps tenodesis, a torn superior labrum does not affect glenohumeral stability and therefore does not require anatomic repair in an overhead throwing athlete. METHODS: Baseline anterior, posterior, and abduction and maximal external rotation glenohumeral translation data were collected from 20 cadaveric shoulders. Translation testing was repeated after the creation of anterior (n = 10) and posterior (n = 10) type II SLAP lesions. Translation re-evaluation after biceps tenodesis was performed for each specimen. Finally, anatomic SLAP lesion repair and testing were performed. RESULTS: Anterior and posterior SLAP lesions led to significant increases in glenohumeral translation in all directions (P < .0125). Biceps tenodesis showed no significance in stability compared with SLAP alone (P > .0125). Arthroscopic repair of anterior SLAP lesions did not restore anterior translation compared with the baseline state (P = .0011) but did restore posterior (P = .823) and abduction and maximal external rotation (P = .806) translations. Repair of posterior SLAP lesions demonstrated no statistical difference compared with the baseline state (P > .0125). CONCLUSIONS: With no detrimental effect on glenohumeral stability in the presence of a SLAP lesion, biceps tenodesis may be considered a valid primary or revision surgery for patients suffering from symptomatic type II SLAP tears. However, biceps tenodesis should be considered with caution as the primary treatment of SLAP lesions in overhead throwing athletes secondary to its inability to completely restore translational stability.

Arthroscopic primary rotator cuff repairs in patients aged younger than 45 years.

PURPOSE: The purpose of this study was to evaluate the mechanism of injury, patient characteristics, tear size, and clinical outcomes after arthroscopic primary rotator cuff repair of full-thickness tears in patients aged younger than 45 years. METHODS: A total of 70 consecutive patients were reviewed in a retrospective, multicenter (2 institutions) study evaluating prospectively collected data. Fifty-three patients, with a mean age of 37.5 years (range, 16.2 to 44.9 years), were available for follow-up at a mean of 35.8 months (range, 13.8 to 59.1 months). Exclusion criteria included patients with revision procedures, repair of partial tears, and follow-up of less than 12 months. Follow-up evaluation included physical examination with dynamometer strength testing and clinical outcome measures including the Single Assessment Numeric Evaluation score, American Shoulder and Elbow Surgeons (ASES) score, Constant-Murley score, pain score on a visual analog scale, and Simple Shoulder Test score. RESULTS: A total of 60% of the patients (32 of 53) had a traumatic etiology, with 38% (12 of 32) of these related to an athletic event. Of the tears, 36 (68%) were medium tears. Concomitant procedures performed at the time of rotator cuff repair included acromioplasty (51), biceps tenodesis or tenotomy (24), distal clavicle excision (10), anteroinferior stabilization (2), and labral repair (1). The mean postoperative ASES score was 84.6 (range, 21.6 to 100.0), with 2 patients recording ASES scores of less than 50 (21.7 and 41.7) at final follow-up. In the 38 patients available for clinical follow-up examination, forward flexion improved from 158.7° (range, 45° to 180°) to 168.4° (range, 120° to 180°) (P = .014). At the time of follow-up, no patients had undergone revision surgery. On the basis of poor clinical outcome scores, 2 patients (4.0%) were considered failures. CONCLUSIONS: Arthroscopic primary rotator cuff repair of full-thickness tears in patients aged younger than 45 years results in improved outcomes with regard to pain, subjective patient satisfaction, and shoulder function. LEVEL OF EVIDENCE: Level IV, therapeutic case series.

Low accuracy of interpretation of rotator cuff MRI in patients with osteoarthritis.

BACKGROUND: Magnetic resonance imaging (MRI) is considered to be a valuable tool for the diagnosis of rotator cuff tears in patients with severe glenohumeral osteoarthritis who are indicated for total shoulder arthroplasty (TSA). We determined the sensitivity, specificity, and positive predictive value of MRI in diagnosing rotator cuff tears in such patients. METHODS: MRI reports of 100 patients who had completed a shoulder MRI prior to TSA were reviewed to determine the radiologists' interpretation of the MRI including the diagnosis, presence of a full-thickness cuff tear, and the presence of atrophy and/or fatty infiltration within the rotator cuff muscle bellies. Operative reports were used as a gold standard to determine whether a full-thickness rotator cuff tear was present. RESULTS: Preoperative MRI reports noted 33 of the 100 patients as having a full-thickness rotator cuff tear, 17 of which had multiple tendon tears. 2 of the 33 patients with full tears on MRI were found to have full-thickness tears at surgery. The sensitivity, specificity, and positive predictive value for MRI detection of full-thickness tears were 100%, 68%, and 6% respectively, with a false-positive rate of 32% and an accuracy of 69%. INTERPRETATION: The study suggests that although MRI is highly sensitive, it has a low positive predictive value and moderately low specificity and accuracy in detecting full-thickness rotator cuff tears in patients with severe glenohumeral osteoarthritis.

Triceps Brachii in Incomplete Tetraplegia: EMG and Dynamometer Evaluation of Residual Motor Resources and Capacity for Strengthening.

BACKGROUND: Candidates for activity-based therapy after spinal cord injury (SCI) are often selected on the basis of manual muscle test scores and the classification of the injury as complete or incomplete. However, these scores may not adequately predict which individuals have sufficient residual motor resources for the therapy to be beneficial. OBJECTIVE: We performed a preliminary study to see whether dynamometry and quantitative electromyography (EMG) can provide a more detailed assessment of residual motor resources. METHODS: We measured elbow extension strength using a hand-held dynamometer and recorded fine-wire EMG from the triceps brachii muscles of 4 individuals with C5, C6, or C7 level SCI and 2 able-bodied controls. We used EMG decomposition to measure motor unit action potential (MUAP) amplitudes and motor unit (MU) recruitment and firing-rate profiles during constant and ramp contractions. RESULTS: All 4 subjects with cervical SCI (cSCI) had increased MUAP amplitudes indicative of denervation. Two of the subjects with cSCI had very weak elbow extension strength (<4 kg), dramatically reduced recruitment, and excessive firing rates (>40 pps), suggesting profound loss of motoneurons. The other 2 subjects with cSCI had stronger elbow extension (>6 kg), more normal recruitment, and more normal firing rates, suggesting a substantial remaining motoneuron population. CONCLUSIONS: Dynamometry and quantitative EMG may provide information about the extent of gray matter loss in cSCI to help guide rehabilitation strategies.

An Accurate and Real-Time Method for Resolving Superimposed Action Potentials in MultiUnit Recordings.

OBJECTIVE: Spike sorting of muscular and neural recordings requires separating action potentials that overlap in time (superimposed action potentials (APs)). We propose a new algorithm for resolving superimposed action potentials, and we test it on intramuscular EMG (iEMG) and intracortical recordings. METHODS: Discrete-time shifts of the involved APs are first selected based on a heuristic extension of the peel-off algorithm. Then, the time shifts that provide the minimal residual Euclidean norm are identified (Discrete Brute force Correlation (DBC)). The optimal continuous-time shifts are then estimated (High-Resolution BC (HRBC)). In Fusion HRBC (FHRBC), two other cost functions are used. A parallel implementation of the DBC and HRBC algorithms was developed. The performance of the algorithms was assessed on 11,000 simulated iEMG and 14,000 neural recording superpositions, including two to eight APs, and eight experimental iEMG signals containing four to eleven active motor units. The performance of the proposed algorithms was compared with that of the Branch-and-Bound (BB) algorithm using the Rank-Product (RP) method in terms of accuracy and efficiency. RESULTS: The average accuracy of the DBC, HRBC and FHRBC methods on the entire simulated datasets was 92.16±17.70, 93.65±16.89, and 94.90±15.15 (%). The DBC algorithm outperformed the other algorithms based on the RP method. The average accuracy and running time of the DBC algorithm on 10.5 ms superimposed spikes of the experimental signals were 92.1±21.7 (%) and 2.3±15.3 (ms). CONCLUSION AND SIGNIFICANCE: The proposed algorithm is promising for real-time neural decoding, a central problem in neural and muscular decoding and interfacing.

Team Approach: Extremity Soft Tissue Sarcoma.

¼ Synovial sarcoma is a soft tissue sarcoma that most commonly presents in the extremity in a periarticular location.¼ As the history and physical examination of patients with synovial sarcoma can overlap considerably with those of patients with non-oncologic orthopedic conditions, it is important that orthopedic surgeons maintain a high level of suspicion when caring for patients with extremity masses.¼ Soft tissue sarcomas are best treated using a team approach. Early recognition and referral to a multidisciplinary sarcoma team are crucial to ensure the best clinical outcome for the patient.

An institution-specific analysis of ACL reconstruction failure.

The purpose of this study was to determine the most common causes of failed anterior cruciate ligament reconstruction (ACLR) using modern reconstructive techniques at a single, high-volume institution. In addition, the clinical outcomes of patients undergoing revision ACLR will be reported. The surgical logs of four senior knee surgeons were retrospectively reviewed for all patients who had undergone ACLR between 2002 and 2009. Patients were excluded if they did not have both the primary and revision surgery on the same knee with the same surgeon. Out of 1944 ACL reconstructions, 28 patients (56 reconstructions) were included in the study. Radiographic studies, operative reports, KT-1000 scores, and chart notes were used to identify all potential factors that may have led to failure. All patients were invited to return for a follow-up examination and survey. Of the 28 patients, the mean age at the index and revision procedure was 22 +/- 11 (range, 12 to 50) and 24 +/- 11 (range, 14 to 57), respectively. In 20 cases, the cause of failure was determined to be acute trauma (sports, work, or accident); in 1 case, the cause was biologic failure; while in 7 cases, the cause was technical error. During the study period the surgeons performed a combined total of 1944 procedures, for an overall failure rate of 1.8%. Twenty patients (71%) were available for follow-up at a mean 30.2 +/- 17.7 months. The overall postrevision outcomes were good to excellent for a majority of patients, with an average Lysholm score of 84 +/- 15.5 and International Knee Documentation Committee score of 77.2 +/- 13.8. The pre- and postoperative KT-1000 scores were 12.1 +/- 2.8 and 6.7 +/- 2.8, respectively. The results from this study suggest that traumatic re-injury, and not surgical/surgeon error, is the most common cause of ACLR failure using anatomic reconstructive principles and strong fixation. In addition, good to excellent outcomes following revision ACLR can be expected in the majority of patients.

Genomic Profiling of Low-grade Intramedullary Cartilage Tumors Can Distinguish Enchondroma From Chondrosarcoma.

Low-grade intramedullary cartilage tumors include enchondroma and grade 1 chondrosarcoma. Classification based on radiopathologic correlation guides treatment, typically observation for asymptomatic enchondroma and surgery for chondrosarcoma. However, some tumors elude classification because radiographic and morphologic findings are equivocal. To date, no ancillary tests are available to aid the diagnosis of such indeterminate or suspicious tumors. We investigated the genomic landscape of low-grade cartilage tumors to determine the profile. We studied 10 each enchondroma, grade 1 chondrosarcoma, and suspicious cartilage neoplasms, respectively, by capture-based next-generation sequencing targeting 479 cancer genes and copy number. In enchondroma, IDH1 or IDH2 hotspot activating mutations and/or COL2A1 alterations were identified in 70% and 60% of cases, respectively; copy number changes were rare (20%). Suspicious cartilage neoplasms had frequent hotspot mutations in IDH1 or IDH2 and alterations in COL2A1 (90% and 70%, respectively); copy number changes were rare (20%). Overall, 80% of suspicious cartilage neoplasms were genomically indistinguishable from enchondroma. In contrast, 20% of chondrosarcoma had IDH1 or IDH2 alterations, 100% demonstrated alteration of COL2A1, and 70% had genomes with numerous copy number gains and losses. In total, 80% of chondrosarcomas demonstrated additional pathogenic mutations, deep deletions, or focal amplifications in cancer genes, predominantly CDKN2A. These results demonstrate distinct genomic profiles of enchondroma and grade 1 chondrosarcoma. Further, sequencing may aid in the correct classification of diagnostically challenging tumors. Additional pathogenic alterations (such as in CDKN2A) or numerous copy number gains or losses would support a diagnosis of chondrosarcoma although the absence of such findings does not exclude the diagnosis.

Rigorous performance assessment of the algorithms for resolving motor unit action potential superpositions.

It is necessary to decompose the intra-muscular EMG signal to extract motor unit action potential (MUAP) waveforms and firing times. Some algorithms were proposed in the literature to resolve superimposed MUAPs, including Peel-Off (PO), branch and bound (BB), genetic algorithm (GA), and particle swarm optimization (PSO). This study aimed to compare these algorithms in terms of overall accuracy and running time. Two sets of two-to-five MUAP templates (set1: a wide range of energies, and set2: a high degree of similarity) were used. Such templates were time-shifted, and white Gaussian noise was added. A total of 1000 superpositions were simulated for each template and were resolved using PO (also, POI: interpolated PO), BB, GA, and PSO algorithms. The generalized estimating equation was used to identify which method significantly outperformed, while the overall rank product was used for overall ranking. The rankings were PSO, BB, GA, PO, and POI in the first, and BB, PSO, GA, PO, POI in the second set. The overall ranking was BB, PSO, GA, PO, and POI in the entire dataset. Although the BB algorithm is generally fast, there are cases where the BB algorithm is too slow and it is thus not suitable for real-time applications.

The effects of a three-week use of lumbosacral orthoses on trunk muscle activity and on the muscular response to trunk perturbations.

BACKGROUND: The effects of lumbosacral orthoses (LSOs) on neuromuscular control of the trunk are not known. There is a concern that wearing LSOs for a long period may adversely alter muscle control, making individuals more susceptible to injury if they discontinue wearing the LSOs. The purpose of this study was to document neuromuscular changes in healthy subjects during a 3-week period while they regularly wore a LSO. METHODS: Fourteen subjects wore LSOs 3 hrs a day for 3 weeks. Trunk muscle activity prior to and following a quick force release (trunk perturbation) was measured with EMG in 3 sessions on days 0, 7, and 21. A longitudinal, repeated-measures, factorial design was used. Muscle reflex response to trunk perturbations, spine compression force, as well as effective trunk stiffness and damping were dependent variables. The LSO, direction of perturbation, and testing session were the independent variables. RESULTS: The LSO significantly (P < 0.001) increased the effective trunk stiffness by 160 Nm/rad (27%) across all directions and testing sessions. The number of antagonist muscles that responded with an onset activity was significantly reduced after 7 days of wearing the LSO, but this difference disappeared on day 21 and is likely not clinically relevant. The average number of agonist muscles switching off following the quick force release was significantly greater with the LSO, compared to without the LSO (P = 0.003). CONCLUSIONS: The LSO increased trunk stiffness and resulted in a greater number of agonist muscles shutting-off in response to a quick force release. However, these effects did not result in detrimental changes to the neuromuscular function of trunk muscles after 3 weeks of wearing a LSO 3 hours a day by healthy subjects.