Stiffness, strength and healing assessment in different bone fractures--a simple mathematical model

External fixators can only be removed safely when fractures have healed sufficiently to restore mechanical integrity to the bone. A bending stiffness of 15 N m/° has been suggested as a means of estimating mechanical integrity. To examine whether this end point stiffness value can be applied to all fractures, the present study examined the degree of variability in predicted stiffness and strength that arises from variations in bone dimensions.

Results imply that there is no common value for the end-point of bending stiffness in different bones. At an end point value of 15 N m/°, the maturity of the fracture repair tissue (represented by its elastic modulus) can vary 500-fold between an adult femur with a 0.5-mm gap to a child's mid diaphyseal tibia with a 1.0-mm gap. Fortunately, the strength does not vary by as large an extent as the modulus. However, even though two fractures each have reached a stiffness of 15 N m/°, a fracture in a bone of 50 mm diameter may exhibit only 60% of the strength of repair in a bone of 30 mm diameter. Therefore, caution should be exercised when using the bending stiffness as an end point indicator for different bones.     

Circular External Fixation Frames with Divergent Half Pins: A Pilot Biomechanical Study

The use of hexapod circular external fixators has simplified the ability to correct complex limb deformities without cumbersome frame reconfigurations. These frames are applied primarily using half pin mountings and may be difficult to utilize given the constraints of traditional half pin constructs. We compared the biomechanical performance of simplified divergent half pin frames to mountings currently being utilized for application of hexapod frames. Three 6-mm half pins per limb segment were placed into sawbones at 60° divergent angles in both the sagittal and coronal planes in a 2-cm diaphyseal fracture gap model. Pin mountings were attached to a standardized four-ring construct. This was compared to similar four-ring frames with two differing pin/wire configurations: (1) two tensioned wires per ring placed at 90° angles, a total eight wires; and (2) two 5-mm half pins per ring placed at 90° angles, a total eight half pins. The divergent 6-mm half pin frames demonstrated similar performance compared the standardized tensioned wire and 5-mm half pin frames in terms of axial micromotion and angular deflection. Based on the mechanical performance of these divergent half pin frames we believe they can be used clinically without detrimental consequences. One or more of the authors (JTW) has received funding from Smith & Nephew.     

Biomechanic comparison of the Teno Fix tendon repair device with the cruciate and modified Kessler techniques

PURPOSE: To compare the mechanical behavior of a novel internal tendon repair device with commonly used 2-strand and 4-strand repair techniques for zone II flexor tendon lacerations. METHODS: Thirty cadaveric flexor digitorum profundus tendons were randomized to 1 of 3 core sutures: (1) cruciate locked 4-strand technique, (2) modified Kessler 2-strand core suture technique, or (3) Teno Fix multifilament wire tendon repair device. Each repair was tested in the load control setting on a Instron controller coupled to an MTS materials testing machine load frame by using an incremental cyclic linear loading protocol. A differential variable reluctance transducer was used to record displacement across the repair site. Cyclic force (n-cycles) to 1-mm gap and repair failure was recorded using serial digital photography. RESULTS: There was no significant difference in differential variable reluctance transducer displacement between the cruciate, modified Kessler, and Teno Fix repairs. The cruciate repair had greater resistance to visual 1-mm repair-site gap formation and repair-site failure when compared with the Kessler and Teno Fix repairs. No significant difference was found between the modified Kessler repair and the Teno Fix repair. In all specimens, the epitenon suture failed before the core suture. Repair failure occurred by suture rupture in the 7 cruciate specimens that failed, with evidence of gap formation before failure. Seven of 10 modified Kessler repairs failed by suture rupture. All of the Teno Fix repairs failed by pullout of the metal anchor. CONCLUSIONS: The Teno Fix repair system did not confer a mechanical advantage over the locked cruciate or modified Kessler suture techniques for zone II lacerations in cadaveric flexor tendons during cyclic loading in a linear testing model. This information may help to define safe boundaries for postoperative rehabilitation when using this internal tendon repair device.     

Effect of gap size on gliding resistance after flexor tendon repair

BACKGROUND: Gap formation is a common complication after flexor tendon repair and is associated with adhesion formation, tendon rupture, and decreased strength. The purpose of this study was to investigate the effect of gap formation on tendon gliding resistance after flexor tendon repair in a human cadaver model. METHODS: Twelve index, middle, and ring fingers from four adult human cadaveric hands were used. Gliding resistance versus excursion between the flexor digitorum profundus tendon and the A2 pulley was first measured in intact tendons. After full laceration, each tendon was repaired with the Pennington suture technique and the gliding resistance was measured again. Then, the repaired tendon (a 0-mm gap) was stretched to form a 1-mm gap, and gliding resistance was remeasured. A magnified video image was used to monitor gap size. This process was repeated to evaluate gap sizes of 2, 3, and 4 mm at the repair site. Peak gliding resistance was determined, and the peak gliding resistance was compared among the groups. RESULTS: No significant difference in peak gliding resistance was detected between repaired tendons without a gap and tendons with a 1-mm gap. Repaired tendons with a 2-mm gap could pass through the A2 pulley; however, peak gliding resistance was significantly higher than that for tendons with a 0 or a 1-mm gap (p < 0.05). When the gap reached > or =3 mm, all tendons caught at the A2 pulley edge, causing a dramatically increased peak gliding resistance. CONCLUSIONS: The presence of a 2-mm gap after flexor tendon repair significantly increased tendon peak gliding resistance (p < 0.05), while a gap of > or =3 mm further increased peak gliding resistance because of catching at the pulley edge.     

The biomechanical analysis of a tendon fixation device for flexor tendon repair

PURPOSE: Stainless steel suture is high in tensile strength but is not widely used in flexor tendon repair because of difficulty with handling and knot tying. The purpose of this study was to examine the biomechanical characteristics of the single-strand multifilament stainless steel Teno Fix device (Ortheon Medical, Winter Park, FL) designed for zone II flexor digitorum profundus (FDP) tendon repair. METHODS: Sixty cadaveric flexor tendons were transected and randomized to receive a Teno Fix or 4-stranded (3-0 or 4-0 braided polyester) suture repair; all repairs were tested with and without a 5-0 monofilament polypropylene circumferential epitendinous suture. By using a material testing system all tendons were tested to failure in tension using a linear model with a loading rate of 1 mm/s. Stiffness, force, and energy at both 2-mm gap and peak force were calculated from the resulting force-displacement curves. RESULTS: The 2-mm gapping force was significantly greater for the Teno Fix and the 3-0 repairs than for the 4-0 repairs. The energy absorbed up to 2-mm gap was significantly greater for the Teno Fix, however, than for all suture repairs both with and without a circumferential suture. There was no statistically significant difference in peak force or energy absorbed at peak force between the Teno Fix and suture repairs; the average gap at peak force for all repairs was 5.2 mm. The addition of a circumferential suture increased the 2-mm gapping and peak forces of the Teno Fix repair to 54.5 N and 66.7 N, respectively. CONCLUSIONS: Increased strength and energy absorbed at 2-mm gap and ease of installation makes the Teno Fix a promising repair method.     

Additive enhancement of implant fixation following combined treatment with rhTGF-beta2 and rhBMP-2 in a canine model

BACKGROUND: Gaps at the interface between implant and bone increase the risk of diminished implant fixation and eventual loosening. The purpose of the present study was to determine if combined use of recombinant human transforming growth factor-beta 2 (rhTGF-beta2) and bone morphogenetic protein 2 (rhBMP-2) led to greater implant fixation strength in the presence of interface gaps than the use of either growth factor alone. METHODS: Twenty-eight skeletally mature adult male dogs received one porous-coated titanium implant in the proximal part of each humerus, for a total of fifty-six implantation sites. Spacers were used to establish an initial 3-mm gap between the implant and the host bone at all fifty-six sites. Forty-two implants were coated with hydroxyapatite-tricalcium phosphate and were used in three growth-factor-treatment groups in which the implants placed in the left humerus were loaded with 12 microg of rhTGF-beta2 (Group 1, seven animals), 25 microg of rhBMP-2 (Group 2, seven animals), or 12 microg of rhTGF-beta2 combined with 25 microg of rhBMP-2 (Group 3, seven animals). In these animals, the twenty-one implants that were placed in the right humerus were loaded with buffer only to serve as contralateral controls. In Group 4 (seven animals), the implants were not coated with hydroxyapatite-tricalcium phosphate, the gap in the left humerus was lightly packed with autogenous bone graft, and the gap in the right humerus was left empty to serve as a contralateral control. All animals were killed at twenty-eight days. The primary end points included three mechanical variables: fixation strength, interface stiffness, and energy to failure. Secondary end points included bone ingrowth and bone volume and trabecular architecture in the gap and in a region located 2 mm medial to the implantation site. RESULTS: The hydroxyapatite-tricalcium phosphate coating had no effect on implant fixation, bone ingrowth, or bone formation in the 3-mm gap. Individual growth factor treatments led to 2.3 to 3.2-fold increases in fixation strength and stiffness as compared with the values for the contralateral controls (p < 0.05). The combined growth factor treatment led to 5.7-fold increases in fixation strength and stiffness compared with the values for the contralateral controls (p < 0.01). Autogenous bone graft treatment was associated with 4.5 to 6.4-fold increases in implant fixation strength and stiffness as compared with the values for the contralateral controls (p < 0.01). Compared with the relevant contralateral controls, energy to failure was increased 3.5-fold in association with TGF-beta2 alone (p < 0.05), 4.5-fold in association with TGF-beta2 combined with BMP-2 (p < 0.01), and 2.5-fold in association with autogenous bone-grafting. As much as 63% of the variance in the mechanical end points was associated with variance in bone volume and architecture in the 3-mm gap and in the region of interest located 2 mm medial to the implantation site (p < 0.01). CONCLUSIONS: In this animal model, the combined use of TGF-beta2 and BMP-2 led to more secure mechanical fixation of the implant than did the use of either growth factor alone and demonstrated results that were similar to those associated with the use of autogenous bone graft.     

Comparison of pullout strength of small-diameter cannulated and solid-core screws

The purpose of this study was to determine the difference in pullout strength between cannulated and solid-core small-diameter bone screws. Cannulated screws from different manufacturers were compared against solid-core screws with 2.0-mm, 2.4-/2.5-mm, and 3.0-mm diameters. A synthetic material made to simulate bicortical bone was used as the test medium. The screws were extracted under servohydraulic control. There was no statistically significant difference between any of the cannulated and solid-core 2.0-mm screws used in this study (P < .05). In the 2.4-/2.5-mm screw tests, both of the cannulated screw designs had a significantly higher pullout strength when compared with the solid-core screw (P < .05). In the testing of 3.0-mm screw test, 1 of the cannulated screw designs showed a significantly higher pullout strength than the other cannulated and solid-core screws that were tested (P < .05). The results of this study suggest that small-diameter cannulated bone screws are similar in mechanical pullout strength to solid-core screws.     

Assessment of bone union/nonunion in an experimental model using microcomputed technology

BACKGROUND: High-resolution microcomputed tomography (microCT) is one of the most recent technical developments to visualize and quantify primarily cancellous bone. Regarding bone formation, microCT is becoming increasingly important, although its reliability has not yet been evaluated. Our study had two goals: to develop a reproducible nonunion model and to determine the efficacy of microCT for the assessment of bone healing in this model. METHODS: The designed fracture model in the rat simulates secondary fracture healing. After plate fixation to the femur, diaphysis transverse middiaphyseal osteotomy was performed with a reciprocating saw, resulting in a 0.38-mm gap with a defect of bone and periosteum corresponding to the thickness of the blade. Proximally and distally to this gap, the periosteum was preserved. Thus, three separate zones were defined: proximal femur diaphysis with periosteum, gap, and distal femur diaphysis with periosteum. In the nonunion group (NM group), a model of impaired bone healing (nonunion), silicone foil was wrapped around the femur diaphysis to block any influence from surrounding tissue. Coverage of the bone repair site by thigh muscles was designed for a model of bone union (M group). Four weeks postoperatively, callus formation was determined by conventional anterior-posterior and lateral plain radiographs. Ten weeks later, a second x-ray series was done as the clinical standard evaluation method. Afterward, specimens were harvested for microCT examination (two-dimensional and three-dimensional [3D]). Biomechanical testing was carried out to determine fracture healing. RESULTS: Our model is highly reproducible and results in bone nonunion in five out of six cases (83.3%). In determining fracture site, plain radiographs the least reliable method in comparison to the biomechanical testing which is the most accurate reference method. In contrast, microCT (the 3D reconstruction) showed significant correlation (r = 1) to the results assessed by biomechanical testing, whereas microCT was correct in 100%. We found bone healing in five out of six animals in the M group verified by microCT (in accordance to biomechanical data). In the M group, significantly enhanced bone formation (50%) (p = 0.008) was observed within the osteotomy site (i.e. within the gap), but there was no difference in periosteal bone formation between the groups proximally and distally to the gap. Interestingly, we did not find statistically significant differences in mineralization. CONCLUSION: We conclude that microCT with 3D reconstruction is the optimal method diagnostic tool in fracture healing, especially in nonunion. Furthermore, direct coverage of the fracture site by muscle flaps results in a mineralized enhanced bone formation within the osteotomy site (i.e. within the gap). Skeletal muscle coverage hypothetically might have osteogenic augmentation potential, thus being able to prevent pseudoarthrosis.     

Mechanical stress on tensioned wires at direct and indirect loading: a biomechanical study on the Ilizarov external fixator

Background

The biomechanical effect of indirect weight loading with the Ilizarov ring fixator using a weight-bearing platform has not yet been investigated. The problem of wire loosening and breakage occurs more frequently when patients are mobilised with a weight-bearing platform. Therefore, the aim of this research was to compare the influence of direct and indirect weight loading on the tensioned wires.

Method

A universal testing machine (UTS, Germany) was used in this study. A composite tibia model with a standard four-ring Ilizarov fixator and 1.8-mm wires in anatomical position was used to simulate a clinical situation. Wire strain was measured with two strain gauges positioned at the ring–wire interface of each wire. After a standardised 2-mm mid-diaphyseal osteotomy, an axial load of up to 1000 N was applied to the bone; the different methods of weight loading were evaluated in two experimental set-ups.

Results

A higher axial load was necessary to achieve an osteotomy gap closure at indirect loading. Mechanical stress on the tensioned wires was 400% higher on the proximal wires and 250% higher on the distal wires at a maximum axial loading of 1000 N. Mechanical stress remained on the wires in indirect loading, even after bone end contact, and led to excessive stress under higher weight-bearing amounts.

Conclusion

There is a substantial change in the biomechanical characteristics of the Ilizarov ring fixator when mobilising a patient with a weight-bearing platform. The considerable higher mechanical stress on the wires needs to be considered when patients are mobilised with a weight-bearing platform.     

Comparative biomechanic performances of locked cruciate four-strand flexor tendon repairs in an ex vivo porcine model

PURPOSE: To investigate the effects of 3 different locking configurations on repair strength when used in a cruciate four-strand repair. METHODS: Sixty fresh porcine flexor tendons were transected and repaired with cruciate four-strand core suture repairs with 3 different locking configurations: simple locks (a modification of the Pennigton method), circle locks, and cross locks. Half of the repairs in each locking group were reinforced with a peripheral suture. The tendon repairs were subjected to linear load-to-failure testing. Outcome measures were 2-mm gap force and ultimate tensile strength. RESULTS: The cross lock repair had significantly greater 2-mm gap force and ultimate tensile strength than the simple lock repair, both with and without a peripheral suture. The cross lock repair showed significantly greater 2-mm gap force without a peripheral suture and significantly greater ultimate tensile strength with a peripheral suture than the circle lock repair. With peripheral reinforcement, the cross lock cruciate repair had a mean 2-mm gap force of 92 N and ultimate tensile strength of 119 N. The cross lock cruciate repair consistently produced the strongest biomechanic performance in all outcome measures. CONCLUSIONS: Locking configuration influences the biomechanic performance of cruciate four-strand flexor tendon repairs. Our results suggest that the cruciate repair with cross locks is stronger than repairs with simple locks or circle locks. Whether the results of this ex vivo porcine linear model can be translated to the clinical arena is unknown, because the factors of tendon/sheath friction, tendon healing, and compromised tendon viability from the lock were not addressed.     

Repair strength of tendons of varying gliding curvature: a study in a curvilinear model

PURPOSE: This study was done to investigate changes in the strength of the repaired tendons by different curvatures of tendon motion arcs. METHOD: Forty-two fresh-frozen digital flexor tendons were divided into 4 groups and were repaired by the modified Kessler method with a running peripheral suture. The tendons were pulled over pulleys with tension at a fixed angle of 90 degrees, but radii of gliding curvature of the pulleys were set at 2.0, 1.5, and 1.0 cm in 3 groups, respectively. The tendons in the other group were subjected to linear tension. These tendons were tested in an Instron tensile machine to determine the 2-mm gap formation force and ultimate strength of the repairs. RESULTS: The 2-mm gap formation force of the tendons pulled over the curvature of a 2.0-, 1.0-, and 0.5-cm radius was, respectively, 69%, 61%, and 49% of that pulled linearly. The ultimate strength of the tendons over curvatures of 2.0-, 1.5-, and 1.0-cm radius was, respectively, 77%, 73%, and 63% of that of tendons pulled with linear tension. The gap formation force and ultimate strength were statistically the lowest in the tendons with a gliding radius of curvature of 1.0 cm. Both the gap formation force and the ultimate strength of tendon repairs decreased as the radius decreased. CONCLUSIONS: The curvature of tendon motion arcs affects the repair strength of the tendons. The findings suggest an increased likelihood of repair ruptures in the tendons that glide along curvatures over the sheaths, pulleys, or joints.     

The effect of platelet-rich plasma on bone healing around implants placed in bone defects treated with Bio-Oss: a pilot study in the dog tibia.

OBJECTIVE: The aim of this study was to examine the influence of platelet-rich plasma (PRP) used as an adjunct to Bio-Oss for the repair of bone defects adjacent to titanium dental implants. STUDY DESIGN: In 6 mongrel dogs, 12 screw-shaped titanium dental implants were inserted into the osteotomy sites in the dogs' tibias. Before implantation, a standardized gap (2.0 mm) was created between the implant surface and the surrounding bony walls. The gaps were filled with either Bio-Oss cancellous granules alone or Bio-Oss cancellous granules mixed with PRP. RESULTS: After 4 months, the Bio-Oss-treated defects revealed a significantly higher percentage of bone-implant contact than the defects treated with Bio-Oss and PRP (60.1% vs. 30.8%; P < .05). CONCLUSION: The results indicate that when PRP is used as an adjunct to Bio-Oss in the repair of bone defects adjacent to titanium dental implants, PRP may decrease periimplant bone healing.     

Correlation of bone tunnel diameter with quadrupled hamstring graft fixation strength using a biodegradable interference screw

Purpose: The purpose of this study was to determine whether the ultimate load at failure of a quadrupled hamstring tendon graft (QHT) fixed with a biodegradable interference screw is improved with a more precise match of the bone tunnel diameter to the diameter of the QHT. Type of Study: Biomechanical testing. Methods: In group A, 8 cadaver knees with a mean age of 69.4 years (range, 60 to 76) were used. QHT graft diameters were measured using sleeves in standard 1.0-mm increments, with matching bone tunnels drilled in 1.0-mm increments. In group B, 9 cadaver knees, with a mean age of 66.5 (53 to 81) were used. Grafts were measured using sleeves in 0.5-mm increments and matching bone tunnels in 0.5-mm increments were drilled. In both groups, the QHT grafts were fixed with a biodegradable interference screw (BioScrew, Linvatec, Largo, FL) in both the tibia and the femur. Tendon interference fixation was tested to failure using a material testing device that tensioned the grafts directly in line with the bone tunnels. Bone mineral density was measured using dual photon absorptimetry for the metaphyseal area of the tibias and femora in the area of interference screw fixation. Results: Femoral maximum load at failure significantly improved from 341 N in the 1.0-mm group to 530 N (P <.05) in the 0.5-mm group; the tibial maximum load at failure improved from 221 N to 308 N (P =.35). Conclusions: Fixation strength results of this study suggest that commercially available instrumentation could be improved with sleeves and reamers available in 0.5-mm increments.Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 18, No 8 (October), 2002: pp 901–907     

Gliding characteristics and gap formation for locking and grasping tendon repairs: a biomechanical study in a human cadaver model

PURPOSE: The purpose of this study was to compare the frictional characteristics and mechanical properties of various locking and grasping suture techniques in a human in vitro model of flexor tendon repair. METHODS: Forty-five cadaveric human flexor digitorum profundus tendons were transected in zone II and repaired using 1 of 5 core suture methods (n = 9 per group): either grasping (modified grasping Kessler, modified Lee) or locking (Pennington, modified Pennington, locking Lee) loop suture techniques. All repairs used 4-0 Supramid looped core suture and an epitenon running suture of 6-0 nylon. Gliding resistance at the tendon-pulley interface was measured along with failure strength and gap formation. The force to produce 0.5-, 1.0-, 1.5-, and 2.0-mm gaps were measured. RESULTS: One of the locking repairs, the locking Lee, had a gliding resistance significantly higher than that of one of the grasping repairs (modified grasping Kessler) and the other 2 locking repairs (Pennington, modified Pennington) (p <.05). There was no significant difference between the other grasping (modified Kessler, modified Lee) and locking (Pennington, modified Pennington) suture configurations (p =.21). The maximum force of one of the locking repairs, the modified Pennington repair (48.0 N; standard deviation, 3.9) was significantly higher than the other locking and grasping repairs (p <.05). The force required to produce more than 1.5 mm of gap for the modified Pennington repair was also significantly higher than that for some of the other grasping (modified Kessler, modified Lee) and locking (Pennington) repairs (p <.05). CONCLUSIONS: The lack of significant difference in gliding resistance among the similarly designed modified grasping Kessler, Pennington, and modified Pennington repairs (overall mean, 0.87 N; standard deviation, 0.16) suggests that the locking loop configuration itself does not adversely affect tendon gliding resistance. The modified Pennington repair increased not only ultimate strength but also resistance to gap formation more than 1.5 mm.     

Mechanical and structural characteristics of the new BONE-LOK cortical-cancellous internal fixation device

The purpose of this study was to evaluate the structural and mechanical characteristics of a new and unique titanium cortical-cancellous helical compression anchor with BONE-LOK (Triage Medical, Inc., Irvine, CA) technology for compressive internal fixation of fractures and osteotomies. This device provides fixation through the use of a distal helical anchor and a proximal retentive collar that are united by an axially movable pin (U.S. and international patents issued and pending). The helical compression anchor (2.7-mm diameter) was compared with 3.0-mm diameter titanium cancellous screws (Synthes, Paoli, PA) for pullout strength and compression in 7# and 12# synthetic rigid polyurethane foam (simulated bone matrix), and for 3-point bending stiffness. The following results (mean +/- standard deviation) were obtained: foam block pullout strength in 12# foam: 2.7-mm helical compression anchor 70 +/- 2.0 N and 3.0-mm titanium cancellous screws 37 +/- 11 N; in 7# foam: 2.7-mm helical compression anchor 33 +/- 3 N and 3.0-mm titanium cancellous screws 31 +/- 12 N. Three-point bending stiffness, 2.7-mm helical compression anchor 988 +/- 68 N/mm and 3.0-mm titanium cancellous screws 845 +/- 88 N/mm. Compression strength testing in 12# foam: 2.7-mm helical compression anchor 70.8 +/- 4.8 N and 3.0-mm titanium cancellous screws 23.0 +/- 3.1 N, in 7# foam: 2.7-mm helical compression anchor 42.6 +/- 3.2 N and 3.0-mm titanium cancellous screws 10.4 +/- 0.9 N. Results showed greater pullout strength, 3-point bending stiffness, and compression strength for the 2.7-mm helical compression anchor as compared with the 3.0-mm titanium cancellous screws in these testing models. This difference represents a distinct advantage in the new device that warrants further in vivo testing.     

Treatment of nonunions following anterior cervical discectomy and fusion with interspinous wiring and bone grafting

Between 1979 and 2000, 25 posterior arthrodeses were performed for repair of symptomatic nonunions following anterior discectomy and fusion. The surgical technique in all cases was an interspinous wiring with an autogenous bone graft in 20 and morselized allograft bone in five. The average follow-up was 5 years (range, 1-14 years). In all patients, the anterior pseudarthrosis progressed to a solid union as judged by an independent musculoskeletal radiologist. In 17 patients, their preoperative pain was relieved; however, of these, six developed recurrent pain caused by degeneration at an adjacent level on an average of 5 years (range, 1-14 years) after their posterior surgery. The authors propose that posterior interspinous wiring with a bone graft is a safe and efficacious method of repair of an anterior pseudarthrosis. However, patients must be cautioned that even though the pseudarthrosis can be predictably repaired by a posterior procedure, preoperative pain may not be relieved.     

Zone I flexor digitorum profundus repair: an ex vivo biomechanical analysis of tendon to bone repair in cadavera

PURPOSE: Biomechanical studies of standard flexor digitorum profundus (FDP) tendon to bone repairs show ultimate strengths greater than the applied loads of early motion rehabilitation protocols. Strain data, however, indicate the potential for significant repair site gapping under these physiologic loads. Gaps in excess of 3 mm have been shown to prevent the time accrual of strength in midsubstance tendon repairs and may prevent the restoration of the normal architecture of the tendon-bone interface. Improving the time-zero tensile properties of FDP insertion site repairs may help obviate these issues and improve clinical outcomes. The purpose of this study was to evaluate the ex vivo biomechanical properties of 2 new repair techniques in comparison with the standard FDP tendon to distal phalanx cortical surface repair. METHODS: Thirty human cadaver FDP tendons were released from their insertion sites by sharp dissection and repaired to bone using 1 of 3 repair techniques. Load to failure testing was performed with a servohydraulic materials-testing system (model 8500R; Instron, Canton, MA) analyzing ultimate force, strain at 20 N, rigidity, force to 2-mm gap formation, and displacement at failure. RESULTS: The results of the failure tests indicate that repairs performed with the addition of a peripheral suture had a greater ultimate force, had increased resistance to gap formation, and had increased rigidity and decreased strain at 20 N compared with the tunnel-only and volar cortical surface to the distal phalanx repairs. Although there were no statistically significant differences in ultimate force or rigidity between the tunnel-only and volar cortical surface repairs, the tunnel-only repairs showed lower strain values and increased values for resistance to 2-mm gap formation when compared with the volar cortical surface repairs. There were no differences among any of the repair groups with regard to the magnitude of tendon displacement from the repair site at failure. CONCLUSIONS: The addition of a peripheral suture to the FDP tendon to bone tunnel repair construct improves the time-zero tensile properties as evidenced by statistically significant increases in ultimate force, rigidity, and resistance to gap formations of 2 mm. In comparison with a volar cortical surface repair, the bone tunnel-only repairs were effective at decreasing the amount of repair site strain during applied loads of 20 N. If these improved time-zero tensile properties persist during the early stages of healing, they may help decrease the incidence of repair-site gap formation associated with the forces of early motion rehabilitation protocols.     

Mechanobiology of initial pseudarthrosis formation with oblique fractures.

Mechanical stresses play an important role in regulating tissue differentiation in a variety of contexts during skeletal development and regeneration. It has been shown that some intermittent loading at a fracture site can accelerate secondary fracture healing. However, it has not been shown how the stress and strain histories resulting from mechanical loading of a fracture might, in some cases, inhibit normal fracture healing and induce pseudarthrosis formation. In this study, finite element analysis is used to calculate hydrostatic stress and maximum principal tensile strain patterns in regenerating tissue around the site of an oblique fracture. Using a mechanobiologic view on tissue differentiation, we compared calculated stress and strain patterns within the fracture callus to the histomorphology of a typical oblique pseudarthrosis. Tissue differentiation predictions were consistent with the characteristic histomorphology of oblique pseudarthrosis: in the interfragmentary gap. tensile strains led to "cleavage" of the callus; at the ends of both fracture fragments, hydrostatic pressure and tensile strain caused fibrocartilage formation, and, at discrete locations of the periosteum at the oblique fracture ends, mild hydrostatic tension caused bone formation. We also found that discrete regions of high hydrostatic pressure correlated with locations of periosteal bone resorption. When previous findings with distraction osteogenesis are considered with these observations, it appears that low levels of hydrostatic pressure may be conducive to periosteal cartilage formation but high hydrostatic pressure may induce periosteal bone resorption during bone healing. We concluded that tissue differentiation in pseudarthrosis formation is consistent with concepts previously presented for understanding fracture healing, distraction osteogenesis, and joint formation.     

Effects of direction of tendon lacerations on strength of tendon repairs

PURPOSE: We compared the tensile strength of different repair configurations on tendons with oblique and transverse lacerations. METHOD: Seventy-two fresh pig flexor tendons were divided randomly and repaired using the modified Kessler, the cruciate, or the 4-strand Massachusetts General Hospital (MGH) repair methods. The tendons were lacerated either transversely or obliquely. They were repaired with conventional and oblique suture repairs. The 2-mm gap formation force and ultimate strength were determined as biomechanical performance for each repair. RESULTS: The gap formation and ultimate strength of the tendons vary with orientations of tendon lacerations and suture methods. In the tendons repaired with the modified Kessler or the cruciate methods, the 2-mm gap formation and ultimate strength of obliquely cut tendons were significantly lower than those of transversely cut tendons. The obliquely placed modified Kessler or cruciate sutures significantly improved the repair strength in the tendons with an oblique laceration. In the tendons repaired with the MGH method, no statistical differences were found in the repair strength of obliquely and transversely lacerated tendons. CONCLUSIONS: The direction of tendon lacerations affects strength of certain repair configurations. The nonlocking modified Kessler or the cruciate tendon repairs are weakened considerably when the tendon laceration is oblique but their mechanical performance is strengthened by re-orienting the repair strands to lie parallel to the laceration. The cross-locked configuration of the MGH repair is not affected by the obliquity of the tendon laceration.     

Repair process after fibrocartilaginous enthesis drilling: histological study in a rabbit model

Background  Disorders of the enthesis are often a consequence of sports injuries. However, there is uncertainty regarding the process of mechanical stress-related injuries at the enthesis and the subsequent repair process of the injured tissues. To elucidate the repair process of the fibrocartilaginous enthesis, we studied the repair of injured fibrocartilaginous enthesis and the morphological characteristics of the repaired tissue. Methods  We drilled 0.5-mm holes in the right tibial insertion of the patellar tendon of Japanese white rabbits, with their own left sides serving as controls. Specimens harvested at 1, 2, 4, 6, 8, and 12 weeks were examined histologically. Morphologically, the ratios of calcified fibrocartilage-bone interface lengths to enthesial lengths were compared between the control and surgical groups. Results  Repair initiation was observed in the deep bone layer at 1 week, with remarkable progress at 2 weeks. Repair at the enthesis and neoosteogenesis in deep bone layers were detected at 4 weeks, and the drill hole disappeared at 6 weeks. The tendon was partially invaded by fibrocartilage-covered chondroid bone at 8 weeks, and regenerated fibrocartilaginous enthesis and increased calcified fibrocartilage-bone interface irregularity was identified at 12 weeks. The ratios of calcified fibrocartilage-bone interface lengths to enthesial lengths were significantly greater in the surgical group than in the control group. Conclusions  Repair progressed from bone to fibrocartilage and ended at fibrous tissue. Cancellous bone disruption triggered repair in all layers. Removal of the subchondral plate enabled infiltration of nutrients via blood vessels, with the underlying bone acting as a scaffold for the regenerating fibrocartilage.