Alan S. Litsky

A Viscoelastic Biomechanical Model of the Cornea Describing the Effect of Viscosity and Elasticity on Hysteresis

PURPOSE To develop a method for evaluating viscosity and elasticity of the cornea and to examine the effect that both properties have on hysteresis. METHODS A three-component spring and dashpot model was created in Simulink in Matlab to represent the purely elastic and viscoelastic behavior of the cornea during a measurement using device called an ocular response analyzer (ORA). Values for elasticity and viscosity were varied while sinusoidal stress was applied to the model. The simulated stresses were used to determine how hysteresis is affected by the individual components of elasticity, viscosity, and maximum stress. To validate the model, high-speed photography was used to measure induced strain in a corneal phantom during ORA measurement. This measured strain was compared with the strains simulated by the model. RESULTS When the spring in the viscoelastic portion of the model was stiffened, hysteresis decreased. When the spring in the purely elastic element was stiffened, hysteresis increased. If both springs were stiffened together, hysteresis peaked strongly as a function of the viscosity of the viscoelastic element. Below the peak value, lower elasticity was associated with higher hysteresis. Above the peak value, higher elasticity was associated with higher hysteresis. In addition, hysteresis increased as the air maximum pressure was increased. Measurements from phantom corresponded to predictions from the model. CONCLUSIONS A viscoelastic model is presented to illustrate how changing viscosity and elasticity may affect hysteresis. Low hysteresis can be associated with either high elasticity or low elasticity, depending on the viscosity, a finding consistent with clinical reports.

Cannulated Screw Fixation of Jones Fractures: A Clinical and Biomechanical Study

Background Traditional nonsurgical treatment of Jones fractures has high rates of delayed union, nonunion, and refracture. Internal fixation has become the treatment of choice in athletes and active patients. Purpose The purpose of this study was (1) to review the short- and long-term clinical results of cannulated screw fixation of Jones fractures and (2) to perform a biomechanical evaluation of fatigue failure characteristics of several types of screws used in the fixation of Jones fractures. Study Design Retrospective case series and in vitro biomechanical study. Methods Ten male and 5 female patients with Jones fractures fixed with cannulated screws ranging from 4 mm to 6.5 mm in diameter were evaluated by chart review, review of radiographs, and telephone interview. Mean follow-up from surgery to phone survey was 34 months. Screws ranging in size from 2.7 mm to 7.3 mm, both cannulated and noncannulated, stainless steel and titanium, were tested in the laboratory by cyclic loading to 250 N up to a maximum of 200 000 cycles. Results Mean time to healing as shown on radiographs and by full activity after surgery were 7.3 and 7.9 weeks, respectively. All patients were able to return to their previous levels of activity. Screw fatigue data showed that the number of cycles to failure increased with increasing screw diameter. For 4-mm screws, mean number of cycles to failure was 4308 for cannulated titanium screws, 22 012 for cannulated stainless steel screws, and 44 523 for noncannulated stainless steel screws. Conclusions In our patients, cannulated screw fixation of Jones fractures was a procedure that was reliable, had low morbidity, and afforded athletes a quick return to activity. Clinical Relevance The laboratory study suggests that the largest screw possible should be used for surgical fixation of these fractures and that screws less than 4 mm in diameter should be used with caution.

Torso flexion loads and the fatigue failure of human lumbosacral motion segments

Study Design. Spine loads associated with lifting a 9-kg weight were estimated at three torso flexion angles (0°, 22.5°, and 45°), and lumbosacral motion segments were cyclically loaded using these loads until failure or to a maximum of 10,020 cycles. Objectives. To simulate the postures and loads experienced by the lumbar spine during repetitive lifting of moderate weights in different torso flexion postures, and to analyze the fatigue failure response of lumbosacral motion segments. Summary of Background Data. Previous fatigue failure studies of lumbar motion segments have not reproduced the combination of spinal postures, loads, and load rates anticipated in different torso flexion postures during lifting tasks characteristic of those in occupational settings. Methods. Twelve fresh human lumbosacral spines were dissected into three motion segments each (L1–L2, L3–L4, and L5–S1). Motion segments within each spine were randomly assigned to a simulated torso flexion angle (0°, 22.5°, or 45°) using a partially balanced incomplete block experimental design. Spinal load and load rate were determined for each torso flexion angle using previously collected data from an EMG-assisted biomechanical model. Motion segments were creep loaded for 15 minutes, then cyclically loaded at 0.33 Hz. Fatigue life was taken as the number of cycles to failure (10 mm displacement after creep loading). Specimens were inspected to determine failure mechanisms. Results. The degree of torso flexion had a dramatic impact on cycles to failure. Motion segments experiencing the 0° torso flexion condition averaged 8,253 cycles to failure (±2,895), while the 22.5° torso flexion angle averaged 3,257 (±4,443) cycles to failure, and motion segments at the 45° torso flexion angle lasted only 263 cycles(±646), on average. The difference was significant at P < 0.0001, and torso flexion accounted for 50% of the total variance in cycles to failure. Conclusions. Fatigue failure of spinal tissues can occur rapidly when the torso is fully flexed during occupational lifting tasks; however, many thousands of cycles canbe tolerated in a neutral posture. Future lifting recommendations should be sensitive to rapid development of fatigue failure in torso flexion.

Osteogenic gene regulation and relative acceleration of healing by adenoviral‐mediated transfer of human BMP‐2 or ‐6 in equine osteotomy and ostectomy models

This study evaluated healing of equine metatarsal osteotomies and ostectomies in response to percutaneous injection of adenoviral (Ad) bone morphogenetic protein (BMP)‐2, Ad‐BMP‐6, or beta‐galactosidase protein vector control (Ad‐LacZ) administered 14 days after surgery. Radiographic and quantitative computed tomographic assessment of bone formation indicated greater and earlier mineralized callus in both the osteotomies and ostectomies of the metatarsi injected with Ad‐BMP‐2 or Ad‐BMP‐6. Peak torque to failure and torsional stiffness were greater in osteotomies treated with Ad‐BMP‐2 than Ad‐BMP‐6, and both Ad‐BMP‐2‐ and Ad‐BMP‐6‐treated osteotomies were greater than Ad‐LacZ or untreated osteotomies. Gene expression of ostectomy mineralized callus 8 weeks after surgery indicated upregulation of genes related to osteogenesis compared to intact metatarsal bone. Expression of transforming growth factor beta‐1, cathepsin H, and gelsolin‐like capping protein were greater in Ad‐BMP‐2‐ and Ad‐BMP‐6‐treated callus compared to Ad‐LacZ‐treated or untreated callus. Evidence of tissue biodistribution of adenovirus in distant organs was not identified by quantitative PCR, despite increased serum antiadenoviral vector antibody. This study demonstrated a greater relative potency of Ad‐BMP‐2 over Ad‐BMP‐6 in accelerating osteotomy healing when administered in this regimen, although both genes were effective at increasing bone at both osteotomy and ostectomy sites.

Biomechanical Comparison of Coracoclavicular Reconstructive Techniques

Background: Acromioclavicular joint dislocations are common orthopaedic injuries. Numerous operative techniques have been described, but the gold standard has yet to be defined. The goal of fixation is to create a stiff and strong reconstruction of the coracoclavicular ligaments to provide optimal stability. The modified Weaver-Dunn is the traditional surgical procedure. However, due to the high rate of recurrent instability with this technique, a shift toward a more anatomic repair has occurred. Purpose: To evaluate the biomechanical performance of multiple types of coracoclavicular ligament reconstruction. Study Design: Controlled laboratory study. Methods: Thirty fresh-frozen human cadaveric shoulders were assigned to 1 of 5 reconstruction groups or a control group: modified Weaver-Dunn, nonanatomic allograft, anatomic allograft, anatomic suture, and GraftRope. A type III acromioclavicular joint dislocation was simulated in all specimens. The 5 techniques were completed, and a cyclic preload and a load-to-failure protocol were performed. Results: The control had an average load to failure of 1330.6 ± 447.0 N. Compared with all techniques, the anatomic allograft had the highest load to failure, 948 ± 148 N. It had a significantly higher load to failure than the modified Weaver-Dunn (523.2 ± 98.6 N, P = .001), the anatomic suture (578.2 ± 195.3 N, P = .01), the nonanatomic allograft (591.2 ± 65.6 N, P = .003), and the GraftRope (646 ± 167.4, P = .016). No significant difference in load to failure was found between the remaining techniques. Conclusion: The anatomic allograft reconstruction has superior initial biomechanical properties compared with the modified Weaver-Dunn, nonanatomic allograft, anatomic suture, and GraftRope techniques. Clinical Relevance: Anatomic reconstruction of the coracoclavicular ligaments with allograft may provide a stronger biological solution for acromioclavicular joint dislocations. This reconstruction may minimize recurrent subluxation and pain and permit earlier rehabilitation when compared with current techniques.

Pressure distributions on the medial tibial plateau after medial meniscal surgery and tibial plateau levelling osteotomy in dogs

OBJECTIVE To evaluate the effect of medial meniscal release (MMR) and medial, caudal pole hemimeniscectomy (MCH) on pressure distribution in the cranial cruciate ligament (CCL) deficient canine stifle, and with tibial plateau levelling osteotomy (TPLO). ANIMALS Twelve adult dogs. METHODS In experiment one, six pairs of cadaveric canine stifles with an intact CCL were axially loaded with a servo-hydraulic material testing machine and pressure distributions were mapped and quantified using pressure sensitive films. Axial loading of each joint was then repeated following MMR, and again after MCH. In experiment two, six pairs of cadaveric canine stifles with or without TPLO were tested before and after CCL transection, and each MMR and MCH procedure using the same methods of experiment 1. RESULTS In experiment one, MMR and MCH had significant effects on the pressure distribution resulting in a 2.5-fold increase in the percentage of surface area with pressure higher than 10 MPa. In experiment two, CCL transection resulted in a significant change in pressure distribution only in the stifle without TPLO (P<0.05). Both MMR and MCH resulted in a 1.7-fold increase in the percentage of area with peak pressure in the stifle with TPLO (P<0.05). CONCLUSIONS Meniscal surgery results in a change in pressure distribution and magnitude within the medial compartment of the stifle. CLINICAL RELEVANCE Compromised function of the meniscus by either MMR or MCH result in stress concentration which may predispose to osteoarthritis.

Biomechanical Analysis of Distal Biceps Tendon Repair Methods

Background The 1-incision and 2-incision techniques are commonly used methods to repair a distal biceps rupture, and they differ in the location of reinsertion of tendon into bone. Hypothesis The native distal biceps brachii tendon inserts on the posterior-ulnar aspect of the bicipital tuberosity, which functions as a cam, increasing the tendons moment arm during its principal action of forearm supination. Repair of the distal biceps tendon to the anterior aspect of the tuberosity compromises forearm supination due to absence of the bicipital tuberositys cam effect. Study Design Controlled laboratory study. Methods Eleven matched pairs of fresh-frozen cadaveric upper extremities were prepared for repair of the distal biceps tendon using either anterior or posterior reattachment with transosseous suture fixation. Specimens were tested on a materials testing machine with intact distal biceps insertion and after repair. A load cell at the distal radial-ulnar joint measured resultant elbow flexion and forearm supination torque produced by 100-N force applied to the proximal aspect of the tendon. Results Although there was a trend (P = .104) toward loss of supination torque with the anterior reconstruction method, no significant differences in torque (0.80 vs 0.89 N·m) or flexion force (11.87 vs 12.07 N) were found between the anterior and posterior reconstruction techniques. Conclusion There is no statistically significant difference in flexion force or supination torque between the anterior and posterior reconstruction techniques. Clinical Relevance This study supports existing limited clinical data suggesting no functional differences exist between 2 common repair methods. Further biomechanical and clinical investigations directly comparing the results of distal biceps tendon repairs made to the anterior aspect versus the posterior aspect of the tuberosity are necessary to definitely determine if differences exist in resultant elbow flexion and forearm supination functions.

Biomechanical Analysis of Flat and Oblique Tibial Tubercle Osteotomy for Recurrent Patellar Instability

Both flat (Elmslie-Trillat) and oblique (Fulkerson) osteotomy techniques are successful in treating patellar instability episodes by moving the tibial tubercle medially. The oblique osteotomy also results in anterior displacement that decreases patellofemoral forces. Recent reports have described proximal tibial fractures occurring during early weightbearing after oblique osteotomy. We performed oblique and flat osteotomies on 13 pairs of fresh-frozen cadaveric knees. The knees were then tested to failure on a materials testing system by exerting a load through the quadriceps tendon at a rate of 1000 N/sec to simulate a stumble injury. The failure mechanism for flat osteotomies was more likely to be tubercle “shingle” fracture, while oblique osteotomies more frequently failed through a tibial fracture or fixation failure in the posterior tibial cortex. Mean load to failure was significantly higher in the flat osteotomy specimens (1639 N versus 1166 N), as was total energy to failure (224 N m versus 127 N m). There was no significant difference in stiffness (87 N/cm versus 74 N/cm). We recommend the flat osteotomy for patients with isolated recurrent patellar instability and the oblique osteotomy in patients who have concomitant patellofemoral pain or articular degenerative changes. When an oblique osteotomy is used, we recommend postoperative brace protection and restricted weightbearing until the osteotomy heals.

Articular Cartilage Optical Properties in the Spectral Range 300-850 nm.

Measurements of absolute total reflectance were recorded from weight-bearing (n=9) and nonweight-bearing (n=9) equine articular cartilage specimens from 300 to 850 nm using a spectrophotometer with integrating sphere attachment. Following correction of measured spectra for interfacial reflections and edge losses, Kubelka-Munk theory was applied to estimate absorption and scattering coefficient, one-dimensional light intensity distribution, and light penetration depth. Kubelka-Munk absorption coefficients ranged from ∼7 cm-1 at 330 nm to ∼1 cm-1 at 850 nm. A localized absorption peak was noted at ∼340 nm. Above 510 nm, weight-bearing cartilage demonstrated significantly higher absorption coefficients than nonweight-bearing tissue (paired t-test, p<0.05). Kubelka-Munk scattering coefficients ranged from ∼40 cm-1 at 360 nm to ∼6 cm-1 at 850 nm. No statistical differences in scattering coefficient were noted between weight-bearing and nonweight-bearing tissue. Penetration depths predicted by Kubelka-Munk theory ranged from 0.6 mm at 350 nm to over 3 mm at 850 nm. Stronger absorption in weight-bearing cartilage compared to nonweight-bearing tissue resulted in lower light penetration depths in weight-bearing cartilage at all wavelengths longer than 510 nm.

Dermal fibroblast‐mediated BMP2 therapy to accelerate bone healing in an equine osteotomy model

This study evaluated healing of equine metacarpal/metatarsal osteotomies in response to percutaneous injection of autologous dermal fibroblasts (DFbs) genetically engineered to secrete bone morphogenetic protein‐2 (BMP2) or demonstrate green fluorescent protein (GFP) gene expression administered 14 days after surgery. Radiographic assessment of bone formation indicated greater and earlier healing of bone defects treated with DFb with BMP2 gene augmentation. Quantitative computed tomography and biomechanical testing revealed greater mineralized callus and torsional strength of DFb‐BMP2‐treated bone defects. On the histologic evaluation, the bone defects with DFb‐BMP2 implantation had greater formation of mature cartilage and bone nodules within the osteotomy gap and greater mineralization activity on osteotomy edges. Autologous DFbs were successfully isolated in high numbers by a skin biopsy, rapidly expanded without fastidious culture techniques, permissive to adenoviral vectors, and efficient at in vitro BMP2 protein production and BMP2‐induced osteogenic differentiation. This study demonstrated an efficacy and feasibility of DFb‐mediated BMP2 therapy to accelerate the healing of osteotomies. Skin cell‐mediated BMP2 therapy may be considered as a potential treatment for various types of fractures and bone defects.