Mark E. Zobitz

Surgical Treatment of Femoroacetabular Impingement: Evaluation of the Effect of the Size of the Resection

BACKGROUND In patients with symptomatic hip impingement, surgical resection of the femoral head-neck junction may improve the range of motion and relieve pain. A risk of this procedure is fracture. We evaluated the amount of resection of the anterolateral aspect of the femoral head-neck junction that can be done safely. METHODS Cadaveric proximal femoral specimens (fifteen matched pairs) were divided into three groups: 10%, 30%, or 50% of the diameter of one femoral neck was removed, and the contralateral femoral neck was left intact to serve as the control. A compressive load was applied directly to the femoral head. Peak load, stiffness, and energy to fracture were compared among the groups. RESULTS The energy to fracture differed significantly (p = 0.0015) among the 10%, 30%, and 50% resection groups. The peak load after the 50% resection was significantly less (p = 0.0025) than that after the 10% or 30% resection. With the numbers available, there was no significant difference in peak load between the 10% and 30% resections. CONCLUSIONS Resection of up to 30% of the anterolateral quadrant of the head-neck junction did not significantly alter the load-bearing capacity of the proximal part of the femur. However, a 30% resection significantly decreased the amount of energy required to produce a fracture. Thirty percent should be considered to be the greatest feasible amount of resection because of the change in the pattern of the femoral head-neck response to axial loads that we observed.

Internal fixation of dorsally displaced fractures of the distal part of the radius. A biomechanical analysis of volar plate fracture stability.

BACKGROUND Volar plate fixation with use of either a locking plate or a neutralization plate has become increasingly popular among surgeons for the treatment of dorsally comminuted extra-articular distal radial fractures. The purpose of the present study was to compare the relative stability of five distal radial plates (four volar and one dorsal), all of which are commonly used for the treatment of dorsally comminuted extra-articular distal radial fractures, under loading conditions simulating the physiologic forces that are experienced during early active rehabilitation. METHODS With use of a previously validated Sawbones fracture model, a dorsally comminuted extra-articular distal radial fracture was created. The fracture fixation stability of four volar plates (an AO T-plate, an AO 3.5-mm small-fragment plate, an AO 3.5-mm small-fragment locking plate, and the Hand Innovations DVR locking plate) were compared under axial compression loading and dorsal and volar bending simulating the in vivo stresses that are generated at the fracture site during early unopposed active motion of the wrist and digits. A single dorsal plate (an AO pi plate) was used for comparison, with and without simulated volar cortical comminution. The construct stiffness was measured to assess the resistance to fracture gap motion, and comparisons were made among the implants. RESULTS The volar AO locking and DVR plates had greater resistance to fracture gap motion (greater stiffness) compared with the volar AO nonlocking and AO T-plates under axial and dorsal loading conditions (p < 0.01), with no significant difference between the AO volar locking and DVR plates. The volar AO locking plate had greater resistance to fracture gap motion than did the volar AO nonlocking plate under axial loading and dorsal bending forces (p < 0.01). The dorsal pi plate had the greatest resistance to fracture gap motion under axial loading and volar and dorsal bending forces (p < 0.01). However, the pi plate was significantly less stable to axial load and dorsal bending forces when the volar cortex was comminuted (p < 0.01). CONCLUSIONS In this model of dorsally comminuted extra-articular distal radial fractures, dorsal pi-plate fixation demonstrated better resistance to fracture gap motion than did the four types of volar plate fixation. The AO volar locking and DVR plates conferred the greatest resistance to fracture gap motion among the four volar plates tested. Volar locking technology conferred a significant increase in resistance to fracture gap motion as compared with nonlocking plate technology.

Effects of the Glenoid Labrum and Glenohumeral Abduction on Stability of the Shoulder Joint Through Concavity-compression: An in Vitro Study

Background: Although the glenohumeral joint is the most mobile articulation of the human body, it is known to exhibit ball-and-socket kinematics. Compression into the glenoid labral concavity keeps the humeral head centered. The purpose of the present study was to determine the effects of joint position on glenohumeral stability through concavity-compression. Methods: Ten cadaveric shoulders were tested. The glenoid was mounted horizontally onto a six-component load-cell while the humerus was clamped to a vertically unconstrained slide. An x-y stage translated the load-cell with the glenoid underneath the humeral head in eight different directions. Compressive loads of 20, 40, and 60 N were applied. The tests were repeated in 0°, 30°, 60°, and 90° of glenohumeral abduction with and without the labrum. Relative translations between the glenoid and the humeral head and the forces resisting translation were recorded. Then the stability ratio, defined as the peak translational force divided by the applied compressive force, was calculated. Results: The average stability ratio was higher in the hanging-arm position than it was in glenohumeral abduction. The highest stability ratio was detected in the inferior direction (59.8% 7.7%) when the labrum was intact and in the superior direction (53.3% 7.9%) when the labrum had been resected. Under both conditions, the anterior direction was associated with the lowest stability ratio (32.0% 4.4% with the labrum and 30.4% 4.1% without the labrum). Resection of the glenoid labrum resulted in an average decrease in the stability ratio of 9.6% 1.7%. With increasing compressive load, the average stability ratio slightly decreased. Conclusions: Glenohumeral stability through concavity-compression was greater in the hanging-arm position than it was in glenohumeral abduction. The average contribution of the labrum to glenohumeral stability through concavity-compression was approximately 10%, about one-half of the value previously reported. With the labrum intact, the glenohumeral joint was most stable in the inferior direction. Without the labrum, it was most stable in the superior direction. Under both conditions, it was least stable in the anterior direction. Glenohumeral joint stability through concavity-compression decreases with higher compressive loads. Clinical Relevance: Anterior dislocation of the shoulder may be facilitated by the lower stability demonstrated in glenohumeral abduction. The labrum may not contribute to glenohumeral stability as much as was previously assumed. However, even moderate compressive forces are sufficient to provide stability through concavity-compression.

Biomechanical comparison of effects of supraspinatus tendon detachments, tendon defects, and muscle retractions.

Background: Rotator cuff ruptures are frequently associated with loss of strength of the shoulder. However, the characteristics of the rotator cuff tear that are responsible for the loss of force generation and transmission have not yet been identified. The purpose of this study was to compare the effects of supraspinatus tendon detachments, tendon defects, and muscle retractions on in vitro force transmission by the rotator cuff to the humerus. Methods: The rotator cuff tendons from ten cadaver shoulders were loaded proportionally to the respective cross-sectional areas of their muscles. A fiberglass rod was cemented into the medullary canal of the humerus and connected to a three-component load cell for the measurement of the forces transmitted by the rotator cuff to the humerus. This study was performed with the humerus in a hanging arm position and with various sizes of supraspinatus tendon detachments, tendon defects, and muscle retractions. Results: Detachment or creation of a defect involving one-third or two-thirds of the supraspinatus tendon resulted in a minor reduction in the force transmitted by the rotator cuff (≤5%), while detachment or creation of a defect involving the whole tendon resulted in a moderate reduction (11% and 17%, respectively). Simulated muscle retraction involving one-third, two-thirds, and the whole tendon resulted in losses of torque measuring 19%, 36%, and 58%, respectively. Side-to-side repair of the one-third and two-thirds defects nearly restored the force transmission capability, whereas a deficit remained after side-to-side repair following complete resection. Conclusions: Our results support the rotator cable concept and correspond to the clinical observation that patients with a small rupture of the rotator cuff may present without a loss of shoulder strength. Muscle retraction is potentially an important factor responsible for loss of shoulder strength following large rotator cuff ruptures. Clinical Relevance: Supraspinatus muscle retraction diminishes glenohumeral abduction torque significantly more than either a defect in the tendon or a simple detachment of the tendon from the tuberosity. In cases of irreparable defects, side-to-side repair may be worthwhile to restore muscle tension and the integrity of the rotator cable.

The effect of knot location, suture material, and suture size on the gliding resistance of flexor tendons.

The effect of knot location, suture material, and suture size on gliding resistance between the pulley and flexor tendon was investigated in a canine model. Different suture materials [monofilament nylon (Ethilon), braided polyester suture coated with silicone (Ticron) and uncoated braided polyester suture (Mersilene)] and suture sizes (4-0, 5-0) were tested. A knot was made on either the volar surface, on one lateral side, or on both lateral sides of canine hind-paw tendons, and gliding resistance was measured. In addition, the frictional coefficient between three suture materials (4-0 nylon, 4-0 Ticron, 4-0 Mersilene) and a nylon rod were measured. The gliding resistance of the tendon with knots on both sides was highest, while tendons with one lateral knot had the lowest resistance (p < 0.01). The gliding resistance of 4-0 suture size was higher than that of 5-0 size (p < 0.0001). The coefficient of friction of nylon was lower than that of braided polyester suture (Ticron or Mersilene) (p < 0.001). The placement of knots and choice of suture material affect gliding resistance after tendon repair, and may, therefore, have an effect on the result of tendon repair.

Snowboarder’s Talus Fracture Mechanism of Injury

Fracture of the lateral process of the talus is an injury unique to snowboarders and is of particular clinical relevance because it masquerades as an anterolateral ankle sprain and is difficult to detect on standard radiographic views. Misdiagnosis can lead to long-term morbidity in a young and active population, with ensuing severe degeneration of the subtalar joint. To date, the precise mechanism of injury has not been established, making it difficult to identify potential preventive strategies in equipment design or snowboarding technique. Fracture of the lateral process of the talus in snowboarders has been thought to result from pure dorsiflexion and inversion combined with axial loading. We hypothesized, however, that external rotation is a key component of the mechanism of injury. Ten cadaveric ankles were mounted on a materials testing machine in a position of fixed dorsiflexion and inversion. All ankles were loaded to failure axially, with or without combined external rotation. No fractures occurred after axial loading in dorsiflexion and inversion, but six of eight specimens sustained fractures of the lateral process of the talus when similarly loaded with external rotation added, supporting our hypothesis. Further study is needed to evaluate the relationship between various types of snowboarding equipment and fracture mechanism.

Mechanical Properties of a Biodegradable Bone Regeneration Scaffold

Poly (Propylene Fumarate) (PPF), a novel, bulk erosion, biodegradable polymer, has been shown to have osteoconductive effects in vivo when used as a bone regeneration scaffold (Peter, S. J., Suggs, L. J., Yaszemski, M. J., Engel, P. S., and Mikos, A. J., 1999, J. Biomater. Sci. Polym. Ed., 10, pp. 363-373). The material properties of the polymer allow it to be injected into irregularly shaped voids in vivo and provide mechanical stability as well as function as a bone regeneration scaffold. We fabricated a series of biomaterial composites, comprised of varying quantities of PPF, NaCl and beta-tricalcium phosphate (beta-TCP), into the shape of right circular cylinders and tested the mechanical properties in four-point bending and compression. The mean modulus of elasticity in compression (Ec) was 1204.2 MPa (SD 32.2) and the mean modulus of elasticity in bending (Eb) was 1274.7 MPa (SD 125.7). All of the moduli were on the order of magnitude of trabecular bone. Changing the level of NaCl from 20 to 40 percent, by mass, did not decrease Ec and Eb significantly, but did decrease bending and compressive strength significantly. Increasing the beta-TCP from 0.25 g/g PPF to 0.5 g/g PPF increased all of the measured mechanical properties of PPF/NVP composites. These results indicate that this biodegradable polymer composite is an attractive candidate for use as a replacement scaffold for trabecular bone.

Gliding characteristics of tendon repair in canine flexor digitorum profundus tendons

The gliding resistance between the flexor digitorum profundus (FDP) tendon and the proximal pulley system was measured using the method of S. Uchiyama, J.H. Coert, L. Berglund, P.C. Amadio, K.N. An (J. Orthop. Res. 13 (1995) 83) in 108 adult dog digits in vitro. The FDP tendons were then lacerated to 80% of their transverse section. Each tendon was repaired with one of the following six suture techniques: Kessler, modified Kessler, Savage, Lee, Becker and simple running suture alone. Each repaired tendon was then tested again using the same method. The Student—Newman—Keuls test for multiple comparisons was performed for statistical analysis. The average gliding resistances of the Kessler, Savage, and Becker repairs were significantly greater than the resistances of the Lee, modified Kessler, and running suture alone repairs (P < 0.05). The Lee suture technique had a significantly greater resistance than the modified Kessler repair and the running suture (P < 0.05). The results of the peak gliding resistance followed the same trends, except that the modified Kessler repair was significantly higher than the running suture alone (P < 0.05). Suture techniques with a multi‐strand core suture, with knots located outside the tendon surface, and with multiple‐loops on the tendon surface may result in increased gliding resistance between the tendon and pulley system after tendon repair.

Surgical treatment of femoroacetabular impingement: evaluation of the effect of the size of the resection. Surgical technique.

BACKGROUND In patients with symptomatic hip impingement, surgical resection of the femoral head-neck junction may improve the range of motion and relieve pain. A risk of this procedure is fracture. We evaluated the amount of resection of the anterolateral aspect of the femoral head-neck junction that can be done safely. METHODS Cadaveric proximal femoral specimens (fifteen matched pairs) were divided into three groups: 10%, 30%, or 50% of the diameter of one femoral neck was removed, and the contralateral femoral neck was left intact to serve as the control. A compressive load was applied directly to the femoral head. Peak load, stiffness, and energy to fracture were compared among the groups. RESULTS The energy to fracture differed significantly (p = 0.0015) among the 10%, 30%, and 50% resection groups. The peak load after the 50% resection was significantly less (p = 0.0025) than that after the 10% or 30% resection. With the numbers available, there was no significant difference in peak load between the 10% and 30% resections. CONCLUSIONS Resection of up to 30% of the anterolateral quadrant of the head-neck junction did not significantly alter the load-bearing capacity of the proximal part of the femur. However, a 30% resection significantly decreased the amount of energy required to produce a fracture. Thirty percent should be considered to be the greatest feasible amount of resection because of the change in the pattern of the femoral head-neck response to axial loads that we observed.

Digital resistance and tendon strength during the first week after flexor digitorum profundus tendon repair in a canine model in vivo.

BACKGROUND After flexor tendon repair, the strength of the repair and the resistance to digital motion are important considerations in deciding when to initiate postoperative rehabilitation. Our objective was to assess these factors in a short-term in vivo canine model of flexor tendon repair. METHODS Forty-eight dogs were randomly allocated to four groups based on the duration of postoperative follow-up (one, three, five, or seven days). In each group, two flexor digitorum profundus tendons of one forepaw were exposed. One tendon (the repair tendon) was sharply transected and repaired with a modified Kessler suture, and the other one (the sham tendon) was simply exposed without laceration. The involved paw was immobilized until the animal was killed on the designated day. Three tendons from each dog, including the repair tendon, the sham tendon, and a control tendon from a corresponding normal digit on the contralateral side, were tested. RESULTS The mean peak total digital resistance force in the repair group was lowest at five days (p < 0.01 compared with seven days; p > 0.05 compared with one and three days). The mean peak force needed to overcome the internal gliding resistance between the repaired tendon and sheath was significantly higher than that in both the sham and control groups at all time-points (p < 0.001); however, this value was also smallest at five days. There was no significant difference in suture strength at any time-point (p > 0.05). CONCLUSIONS When we evaluated tendon-gliding and suture strength after flexor tendon repair, the least favorable ratio of repair strength to force needed to overcome the resistance to digital motion was noted on Day 7, whereas the best combination of tendon strength and low peak resistance force was noted on Day 5.