Neil A. Sharkey

The Rotator Cuff Opposes Superior Translation of the Humeral Head

To determine the influence of rotator cuff muscle activity on humeral head migration relative to the glenoid during active arm elevation we studied five fresh cadaveric shoulders. The shoulder girdles were mounted in an apparatus that simulated contraction of the deltoid and rotator cuff muscles while maintaining the normal scapulothoracic relationship. The arms were abducted using four different configurations of simulated muscle activity: deltoid alone; deltoid and supraspinatus; del toid, infraspinatus, teres minor, and subscapularis; and deltoid, supraspinatus, infraspinatus, teres minor, and subscapularis. For each simulated muscle configura tion the vertical position of the humeral head in relation to the glenoid was determined at 30°, 60°, 90°, and 120° of abduction using digitized anteroposterior radio graphs. Both muscle activity and abduction angle sig nificantly influenced the glenohumeral relationship. With simulated activity of the entire rotator cuff, the geo metric center of the humeral head was centered in the glenoid at 30° but had moved 1.5 mm superiorly by 120°. Abduction without the subscapularis, infraspina tus, and teres minor muscles caused significant supe riorly directed shifts in humeral head position as did ab duction using only the deltoid muscle. These results support the possible use of selective strengthening ex ercises for the infraspinatus, teres minor, and sub scapularis muscles in treatment of the impingement syndrome.

Dynamic Loading of the Plantar Aponeurosis in Walking

BACKGROUND The plantar aponeurosis is known to be a major contributor to arch support, but its role in transferring Achilles tendon loads to the forefoot remains poorly understood. The goal of this study was to increase our understanding of the function of the plantar aponeurosis during gait. We specifically examined the plantar aponeurosis force pattern and its relationship to Achilles tendon forces during simulations of the stance phase of gait in a cadaver model. METHODS Walking simulations were performed with seven cadaver feet. The movements of the foot and the ground reaction forces during the stance phase were reproduced by prescribing the kinematics of the proximal part of the tibia and applying forces to the tendons of extrinsic foot muscles. A fiberoptic cable was passed through the plantar aponeurosis perpendicular to its loading axis, and raw fiberoptic transducer output, tendon forces applied by the experimental setup, and ground reaction forces were simultaneously recorded during each simulation. A post-experiment calibration related fiberoptic output to plantar aponeurosis force, and linear regression analysis was used to characterize the relationship between Achilles tendon force and plantar aponeurosis tension. RESULTS Plantar aponeurosis forces gradually increased during stance and peaked in late stance. Maximum tension averaged 96% +/- 36% of body weight. There was a good correlation between plantar aponeurosis tension and Achilles tendon force (r = 0.76). CONCLUSIONS The plantar aponeurosis transmits large forces between the hindfoot and forefoot during the stance phase of gait. The varying pattern of plantar aponeurosis force and its relationship to Achilles tendon force demonstrates the importance of analyzing the function of the plantar aponeurosis throughout the stance phase of the gait cycle rather than in a static standing position. CLINICAL RELEVANCE The plantar aponeurosis plays an important role in transmitting Achilles tendon forces to the forefoot in the latter part of the stance phase of walking. Surgical procedures that require the release of this structure may disturb this mechanism and thus compromise efficient propulsion.

The Role of the Acetabular Labrum and the Transverse Acetabular Ligament in Load Transmission in the Hip

We performed a biomechanical study of seventeen hip joints in the pelves of nine cadavera in order to assess the role that the acetabular labrum and the transverse acetabular ligament play in load transmission. The distribution of contact area and pressure between the acetabulum and the femoral head was measured with the hip in four different conditions: intact (seventeen hips), after removal of the transverse acetabular ligament (eight hips), after removal of the entire labrum (nine hips), and after removal of both the transverse acetabular ligament and the labrum (seventeen hips). The hip joint was loaded in simulated single-limb stance, and the measurements were made with use of pressure-sensitive film. A peripheral distribution of load was seen in the intact acetabula. This pattern was altered only minimally after removal of the transverse acetabular ligament or the labrum, or both. When both of these structures were removed, the only significant change was a decrease in the maximum pressure in the posterior aspect of the acetabulum (p = 0.02). No significant changes were detected with regard to the contact area, load, mean pressure, or maximum pressure in the anterior or superior aspect of the acetabulum under any testing condition. CLINICAL RELEVANCE: Our findings indicate that removal of the transverse acetabular ligament or the labrum, or both, does not significantly increase pressure or load in the acetabulum and may not predispose the hip to premature osteoarthrosis.

Bone ingrowth and mechanical properties of coralline hydroxyapatite 1 yr after implantation.

A previous study of coralline hydroxyapatite as a bone-graft substitute was extended from 4 to 12 months to determine better the relationships between implantation time, bone ingrowth and mechanical properties. The model consisted of a 10 x 30 mm window defect in the shaft of the canine radius (a cortical site), and a 10 mm diameter cylindrical defect in the head of the humerus (a cancellous site). In the new study, these two defects were made bilaterally in eight dogs, and filled with block-form coralline hydroxyapatite. The radius defects were supported by a metal fixation plate which was removed after 9 months. After 12 months, the dogs were killed and the left-side implants were analyzed histomorphometrically and mechanically. The right-side radius and humerus were reserved for structural analysis. The results were combined with those previously measured after 4, 8, 12 and 16 wk of implantation. In the cortical site, bone ingrowth increased from 52% at 16 wk to 74% at 1 yr. In the cancellous site, bone ingrowth was 38% after 4 wk, then fell monotonically, reaching 17% at 1 yr. Bending and compressive strength and stiffness of the radius implants increased throughout the post-implantation year, but compressive strength and stiffness of the humerus implants did not change after the first 2-4 months. Mechanical properties were strongly correlated to bone ingrowth in the cortical, but not the cancellous, site. The volume fraction of the coralline hydroxyapatite material diminished significantly with time in the cortical, but not the cancellous, site.

The fate of articular cartilage after transplantation of fresh and cryopreserved tissue-antigen-matched and mismatched osteochondral allografts in dogs

The long-term success of massive osteochondral allografts depends not only on the incorporation of the transplanted articular cartilage. Osteochondral allografts are immunogenic, and, once an immune response is stimulated by exposure to donor cellular antigens, the cartilage becomes vulnerable to direct injury by cytotoxic antibodies or by lymphocytes, or to indirect injury by inflammatory mediators and enzymes induced by the immune response. To clarify the role of histocompatibility antigen-matching on the health of transplanted articular cartilage, we orthotopically implanted canine leukocyte antigen-matched and mismatched proximal osteochondral allografts of the radius, both fresh and cryopreserved, in beagles. Four groups of dogs received: (1) canine leukocyte antigen-mismatched frozen allografts, (2) canine leukocyte antigen-mismatched fresh allografts, (3) canine leukocyte antigen-matched fresh allografts, or (4) canine leukocyte antigen-matched frozen allografts. In twelve of the dogs, the contralateral leg was subjected to a sham operation, and in ten of the dogs, the proximal part of the radius was removed and replaced as an autogenous graft control. All animals were followed for eleven months after the operation and then were killed. The cartilage of the grafts was evaluated grossly, histologically, and biochemically. The biochemical analysis consisted of measurement of dry weight, content of glycosaminoglycan and hydroxyproline, and galactosamine-to-glucosamine ratios. Analyses of variance were used to study the effect of tissue antigen-matching and freezing on degradation of cartilage. During the study, no dog had grossly obvious clinical abnormalities, all host-graft interfaces healed, and no joints dislocated. The gross appearance of the cartilage was normal for both the joints that had an autogenous graft and those that were subjected to the sham operation. The cartilage of all allografts was thinned, dull, and roughened. The synovial membrane of all of the joints that had been operated on was mildly fibrotic and hyperplastic, but only that of the dogs that had an allograft was severely fibrotic and hyperplastic and demonstrated an inflammatory response. The inflammatory response was most severe in joints that had received a fresh canine leukocyte antigen-mismatched allograft. Invasive pannus was more frequent in joints that had received a fresh graft, particularly those that had received a canine leukocyte antigen-mismatched allograft, and cartilage was sometimes eroded to subchondral bone. Freezing was harmful to the cartilage. Very few cells survived the freezing procedure, and frozen grafts received s significantly worse histological scores had significantly less glycosaminoglycans and had a lower ratio of galactosamine to glucosamine than fresh grafts.

Anatomic and Biomechanical Assessment of Transarticular Screw Fixation for Atlantoaxial Instability

The purpose of this study is to elucidate anatomically the, atlantoaxial transarticular screw fixation described by Magerl in 1979 and compare it biomechanically with Gallie wiring. Five human C1-C2 specimens were tested in flexion/extension and rotation intact, then after wiring and screw fixation. Mean screw length was 39 mm, 25 mm in the C2 lamina and 14 mm in the lateral mass. Angular displacement of screwed specimens was significantly less than control or wired groups. Stiffness at 0–0.5 Nm loads was significantly greater for screwed specimens than for wired or controls (101 ± 49 Nm, 10.3 ± 9.2 Nm, and 1.96 ± 0.18 Nm, respectively). All specimens withstood 5 Nm in flexion and extension without failure. Screw fixation provides stability comparable to Gallie wiring and is stiffer at low-range forces and rotational angles.

Effects of ovariectomy in beagle dogs

Beagle dogs 3-7 years old were ovariectomized (n = 9) or sham operated (n = 6) and followed for 48 weeks with measurements of body weight, tibial shaft bone mineral content (BMC), and serum biochemistry. Following killing, measurements were made of bone strength and histomorphometry. Ovariectomy (OX) significantly reduced serum estrone and estradiol concentrations and their variability from month to month. There was a transient decrease in cortical BMC of the OX dogs during the first 12 postoperative weeks but no difference between the groups after 48 weeks. Serum osteocalcin was elevated, but there was little effect on serum alkaline phosphatase, Ca, P, or calcitonin. OX increased the number of tetracycline-labeled osteons in cortical bone but reduced the percent trabecular surface labeled with tetracycline. OX produced no significant changes in the composition of the bones or loss of cortical area, but a statistically significant 15% trabecular bone loss occurred in the spine. However, bone strength had not been significantly affected at the time of sacrifice.

Strains in the metatarsals during the stance phase of gait: implications for stress fractures.

BACKGROUND Stress fractures of the metatarsals are common overuse injuries in athletes and military cadets, yet their etiology remains unclear. In vitro, high bone strains have been associated with the accumulation of microdamage and shortened fatigue life. It is therefore postulated that stress fractures in vivo are caused by elevated strains, which lead to the accumulation of excessive damage. We used a cadaver model to test the hypothesis that strains in the metatarsals increase with simulated muscle fatigue and plantar fasciotomy. METHODS A dynamic gait simulator was used to load fifteen cadaveric feet during the entire stance phase of gait under conditions simulating normal walking, walking with fatigue of the auxiliary plantar flexors, and walking after a plantar fasciotomy. Strains were measured, with use of axial strain-gauges, in the dorsal, medial, and lateral aspects of the diaphysis of the second and fifth metatarsals as well as in the proximal metaphysis of the fifth metatarsal. RESULTS When the feet were loaded under normal walking conditions, the mean peak strain in the dorsal aspect of the second metatarsal (-1897 microstrain) was more than twice that in the medial aspect of the fifth metatarsal (-908 microstrain). Simulated muscle fatigue significantly increased peak strain in the second metatarsal and decreased peak strain in the fifth metatarsal. Release of the plantar fascia caused significant alterations in strain in both metatarsal bones; these alterations were greater than those caused by muscle fatigue. After the plantar fasciotomy, the mean peak strain in the dorsal aspect of the second metatarsal (-3797 microstrain) was twice that under normal walking conditions. CONCLUSIONS The peak axial strain in the diaphysis of the second metatarsal is significantly (p < 0.0001) higher than that in the diaphysis of the fifth metatarsal during normal gait. The plantar fascia and the auxiliary plantar flexors are important for maintaining normal strains in the metatarsals during gait.

A dynamic cadaver model of the stance phase of gait: performance characteristics and kinetic validation.

OBJECTIVE: This study was undertaken to evaluate the performance of a new dynamic laboratory model of the stance phase of gait. DESIGN: Five cadaver feet were repetitively tested in the apparatus. BACKGROUND: Typical biomechanical investigations of cadaver feet simply place a static load on the tibia. The present system was designed to better simulate the changing in-vivo loading environment of the foot and ankle during gait. METHODS: The device mimics the behavior of the tibia, foot, and ankle from heel-strike to toe-off by reproducing the physiologic actions of five extrinsic foot muscles and physiologic motion at the proximal tibia. To verify its utility, cadaver gait simulations were conducted while measuring applied muscle forces, ground reaction forces, and plantar pressures. RESULTS: Dynamic muscle forces were consistently delivered to within 10% of pre-programmed values. Dynamic measurements of ground reaction forces and plantar pressure were similar to those measured in healthy human subjects. Peak vertical (y), foreaft (x) and medio-lateral (z) forces were 110, 18, and 4% of body weight respectively. Compressive force in the tibial shaft reached 410% of body weight. RELEVANCE: Cadaver studies have greatly enhanced our understanding of normal and pathologic foot function, but are often limited by over-simplified loading conditions. The apparatus presented here accurately reproduces the in-vivo loading environment and provides a powerful investigational tool for the study of foot and ankle function. With this device, musculoskeletal structures can be examined in detail under biomechanical conditions similar to those they experience in life.

Enhanced osteoclastic resorption and responsiveness to mechanical load in gap junction deficient bone.

Emerging evidence suggests that connexin mediated gap junctional intercellular communication contributes to many aspects of bone biology including bone development, maintenance of bone homeostasis and responsiveness of bone cells to diverse extracellular signals. Deletion of connexin 43, the predominant gap junction protein in bone, is embryonic lethal making it challenging to examine the role of connexin 43 in bone in vivo. However, transgenic murine models in which only osteocytes and osteoblasts are deficient in connexin 43, and which are fully viable, have recently been developed. Unfortunately, the bone phenotype of different connexin 43 deficient models has been variable. To address this issue, we used an osteocalcin driven Cre-lox system to create osteoblast and osteocyte specific connexin 43 deficient mice. These mice displayed bone loss as a result of increased bone resorption and osteoclastogenesis. The mechanism underlying this increased osteoclastogenesis included increases in the osteocytic, but not osteoblastic, RANKL/OPG ratio. Previous in vitro studies suggest that connexin 43 deficient bone cells are less responsive to biomechanical signals. Interestingly, and in contrast to in vitro studies, we found that connexin 43 deficient mice displayed an enhanced anabolic response to mechanical load. Our results suggest that transient inhibition of connexin 43 expression and gap junctional intercellular communication may prove a potentially powerful means of enhancing the anabolic response of bone to mechanical loading.