Representative dynamic ranges of spinal alignment during gait in patients with mild and severe adult spinal deformities.

BACKGROUND CONTEXT: Surgical correction strategies for adult spinal deformity (ASD) relies heavily on radiographic alignment goals, however, there is often debate regarding degree of correction and how static alignment translates to physical ability in daily life. Kinematic analysis has the potential to improve the concept of ideal spinal alignment by providing clinically meaningful estimates of dynamic changes in spinal alignment during activities of daily life. PURPOSE: Estimate representative dynamic ranges of spinal alignment during gait among ASD patients using 3D motion tracking; compare dynamic alignment between mild and severe deformity patients and to healthy adults. STUDY DESIGN/SETTING: Retrospective review at a single institution. PATIENT SAMPLE: Fifty-two ASD patients and 46 healthy adults. OUTCOME MEASURES: Radiographic alignment, kinematic spine motion, spatiotemporal gait measures, patient reported outcomes (VAS pain, ODI, SRS-22r). METHODS: Spinal alignment was assessed radiographically and during standing and overground walking tests. Dynamic alignment was initialized by linking radiographic alignment to kinematic alignment during standing and at initial heel contact during gait. Dynamic changes in maximums and minimums during gait were made relative to initial heel contact for each gait cycle. Total range-of-motion (RoM) was measured for both ASD and healthy subjects. Dynamic alignment measures included coronal and sagittal vertical axes (CVA, SVA), T1 pelvic angle (TPA), lumbar lordosis (LL), and pelvic tilt (PT). ASD patient's deformities were classified as either Mild or Severe based on the SRS-Schwab ASD classification. RESULTS: Severe ASD patients had significantly larger dynamic maximum and minimums for SVA, TPA, LL, and PT (all p<.05) compared with Mild ASD patients. ASD patients exhibited little difference in dynamic alignment compared with healthy subjects. Only PT had a significant difference in dynamic RoM compared with healthy (p<.001). CONCLUSIONS: Mild and Severe ASD patients exhibited similar global dynamic alignment measures during gait and had comparable RoM to healthy subjects except with greater PT and reduced spatiotemporal performance which may be key compensatory mechanisms for dynamic stabilization.

Biomechanics of prophylactic tethering for proximal junctional kyphosis: effects of cyclic loading on tether strength and failure properties.

STUDY DESIGN: Biomechanical evaluation of woven polyester tethers. OBJECTIVES: To quantify changes in tether elongation, stiffness, and failure characteristics after cyclic loading. Ligamentous augmentation is gaining interest as a technique to prevent proximal junctional kyphosis (PJK) in adult spinal fusions. There are a lack of data regarding the effects of cyclic loading on polyester tether mechanical properties. Tether stretch may lead to loss of stabilization and increased risk of tether failure. Biomechanical data are needed to determine the effects of cyclic loading on tether integrity. METHODS: Testing was done in two materials: (1) a synthetic cortical bone composite to determine baseline mechanical properties, and (2) nine cadaveric L1 spinous processes. 5 mm woven polyester tethers were looped through 2.5 mm holes drilled in each material. First, five tethers were tested directly to failure in the synthetic bone to establish baseline failure properties. Next, tethers were tested at one of the three cyclic load ranges [5%, 25%, and 50% (n = 5 each) of baseline failure] for 1000 cycles and then loaded to failure. Cadaveric tests were done at the 25% range and compared to synthetic bone tests at the same range. Cadaveric failure tests were classified as either tether failure or spinous process bone failure. RESULTS: Greater cyclic loading range had a significant effect on tether loop elongation, increased stiffness, and decreased ultimate tensile force. Among the cadaveric failure tests, 56% resulted in tether failure and the remaining 44% resulted in bone failure. CONCLUSIONS: Polyester tethers stretch significantly when loaded to physiological ranges. Anticipation of tether stretch may be an important consideration for a tethering strategy to prevent PJK. Improved understanding of tether material properties can provide guidance for the evaluation of clinical outcomes associated with techniques to reduce the risk of PJK caused by ligamentous laxity. LEVEL OF EVIDENCE: Biomechanical study.

Initial Response of Mature, Experienced Runners to Barefoot Running: Transition to Forefoot Strike.

INTRODUCTION: Forefoot strike has been advocated for many runners because of the relatively lower impact and push-off forces compared to a heel strike. The purpose of this study was to explore the ability of mature (> 30 years old), experienced runners to transition from a heel foot strike to a forefoot strike when first introduced to barefoot running on a treadmill. We hypothesized: 1) mature runners who heel strike while wearing traditional training shoes would persist in heel striking immediately following a switch to barefoot, 2) mean shoe heel-to-toe drop would be significantly greater in runners who persist in heel striking when running barefoot compared to those who transition to a forefoot strike pattern, and 3) there would be a significant decrease in heel striking in the barefoot condition as running speeds increased. METHODS: This was a controlled crossover laboratory study. Thirty-three experienced runners (average 23.4 miles per week) with an average age of 45.6 years were recruited for this study. The participants first ran in their standard running shoes and subsequently barefoot. A motion capture system was utilized to detect and analyze any transition from heel strike to forefoot strike made by study participants. RESULTS: Of the 26 participants who were classified as heel strike runners in their running shoes, 50% (13/26, p = 0.001) transitioned to forefoot strike when changing from running in shoes to running barefoot. CONCLUSIONS: The injuries associated with transition from standard running shoes to barefoot running or minimalist shoes may be influenced by the persistence of heel striking in mature runners. Older experienced runners may have limited ability to transition from heel to forefoot striking when first introduced to barefoot running. Mature runners should be cautious when beginning a minimalist shoe or barefoot running regimen.

Effect of Ultrasound Frequency and Treatment Duration on Antibiotic Elution from Polymethylmethacrylate Bone Cement.

INTRODUCTION: The objective of this study was to evaluate the effect of ultrasound frequency and treatment duration on antibiotic-impregnated polymethylmethacrylate (PMMA) antibiotic elution rates and mechanical strength. METHODS: Two batches of PMMA were prepared: one with five grams of vancomycin powder and one without. Each batch was divided into two frequency groups: kHz and MHz. Each frequency group was divided into two duration groups: two minutes and ten minutes. Elution samples were measured daily using flow injection analysis. After one week of elution, ultrasound treatments were done daily until each group's average concentration fell below those of non-ultrasound control groups. After elution testing, compression testing determined mechanical properties. Paired t-tests were used to compare daily elution amounts to baseline values. Univariate ANOVAs were used to test for effects of both frequency and treatment duration on antibiotic elution amounts and on mechanical properties. RESULTS: All ultrasound treatments resulted in significant increases in antibiotic elution. Frequency and duration had significant effects of increasing antibiotic elution (p < 0.001). The kHz group produced significantly greater antibiotic elution than the MHz group (p < 0.001). The 10-minute duration produced significantly greater antibiotic elution than the two-minute duration (both p < 0.001). Frequency and duration did not have significant effects on yield stress (p = 0.841 and p = 0.179, respectively). Frequency had a significant effect (p = 0.024) on modulus, but duration did not (p = 0.136). CONCLUSIONS: Ultrasound frequency and treatment duration significantly affect antibiotic elution from PMMA which may be helpful for treatment of periprosthetic joint infections during revision arthroplasty.

Biomechanics of Prophylactic Tethering for Proximal Junctional Kyphosis: Characterization of Spinous Process Tether Pretensioning and Pull-Out Force.

STUDY DESIGN: Biomechanical evaluation of cadaver functional spinal units (FSUs). OBJECTIVES: Demonstrate the effect of increasing spinous process (SP) tether pretension on FSU flexion range of motion (ROM), intervertebral disc (IVD) pressure, and SP force. Quantify SP tether pull-out forces and relate them to SP forces generated at maximum flexion. SUMMARY OF BACKGROUND DATA: There has been recent interest in the use of SP tethering for prophylactic treatment of proximal junctional kyphosis (PJK). There is currently no consensus on standard tethering technique and no biomechanical data on the effect of tether pretension. METHODS: Nine T11-T12 FSUs were tested to 5 Nm of flexion-extension bending. A strain gauge was applied at the base of the T11 SP to measure force. Two custom pressure sensors were inserted into the anterior and posterior thirds of the IVD. Motion kinematics were measured by a motion capture system. An untethered test was done to describe baseline behavior. A 5-mm polyester tether was looped through holes drilled at the base of each SP and pretensioned to five different pretensions ranging from 0 to 88 N. Following ROM testing, specimens were dissected into individual vertebra and then SP pull-out testing was done at each level. RESULTS: Increasing pretension significantly reduced flexion ROM, reduced IVD pressures, and increased SP force. All pretensions, including the minimum, significantly reduced flexion ROM. SP pull-out forces were significantly greater than SP forces generated at maximum flexion. CONCLUSIONS: Tether pretension significantly affects segmental FSU biomechanics. Pretension should be considered an integral factor in the overall success of a tethering strategy. Efforts should be made to control and record pretension intraoperatively. LEVEL OF EVIDENCE: Level V, biomechanical study.

Biomechanics of Prophylactic Tethering for Proximal Junctional Kyphosis: Comparison of Posterior Tether Looping Techniques.

STUDY DESIGN: Biomechanical study. OBJECTIVES: Compare effects of four spinous process (SP) tether looping methods on segmental flexion range of motion (ROM), intervertebral disc (IVD) pressures, and peak tether forces. SUMMARY OF BACKGROUND DATA: SP tethering has been gaining interest as a prophylactic technique to prevent PJK caused by ligamentous laxity in ASD corrective surgery. Several SP tether looping methods have been proposed; however, there is no consensus on appropriate technique. No study has investigated the effect of the tether looping method on segmental biomechanics. METHODS: Nine T1-T4 cadaveric motion segments were tested to 5 Nm of flexion-extension. The uppermost instrumented vertebra (UIV) was located at T3 using standard pedicle screws and fusion rods. A crosslink (CL) was placed inferior to the pedicle screws. A 5-mm polyester tether was looped under the CL at UIV and through holes drilled at the base of UIV + 1 and UIV + 2 SPs. Biomechanical measurements included flexion ROM, IVD pressure, and peak tether forces at UIV/UIV + 1 and UIV + 1/UIV + 2. An untethered test was used for baseline values. Tethered tests included one single-level (SL) method and three double-level (DL) methods: common (CM), chained (CH), and figure-8 (F8). RESULTS: SL yielded significant reductions in flexion ROM at UIV/UIV + 1 (p = .001) and in IVD pressure at UIV/UIV + 1 (p = .007). Choice of DL method had a significant effect on flexion ROM at UIV/UIV + 1 (p = .004) but not at UIV + 1/UIV + 2 (p = .14). Choice of DL method also had a significant effect on IVD pressure at UIV/UIV + 1 (p < .001) but not at UIV + 1/UIV + 2 (p = .311). CM produced the greatest reductions in flexion ROM and IVD pressure, with the lowest peak tether forces among the DL methods. CONCLUSION: Tether looping method significantly alters segmental biomechanics. Tethering with the CM method to UIV + 2 allows for reductions in loads acting on the UIV + 1 SP and posterior ligaments. LEVEL OF EVIDENCE: Level V, biomechanical study.