Fixation of periprosthetic femoral shaft fractures associated with cemented femoral stems: a biomechanical comparison of locked plating and conventional cable plates.

UNLABELLED: To determine which of 2 techniques for the treatment of periprosthetic femoral shaft fractures is of greater stiffness. DESIGN: A laboratory study using 8 pairs of matched, embalmed femurs. METHODS: Femurs implanted with a cemented total hip prosthesis had a simulated periprosthetic femur fracture created distal to the implant. Fractures were fixed with a plate with locked screws or a plate with cables (Ogden construct). Fixation stability was compared in various loading modalities before and after cycling. Failure in torsional loading was then determined. The cement mantle was tested for crack propagation that may have occurred secondary to locked screw insertion and loading. OUTCOME MEASUREMENTS: Fixation stiffness (the ratio of applied load to displacement at the fracture site), torsional strength, mode of failure for each system, and cement mantle evaluation for cracks after screw insertion. RESULTS: Locked plating was stiffer than the Ogden construct in pre- and post-cyclic axial loading and torsion. There was no difference in lateral bending stability or torsional failure loads. CONCLUSIONS: Locked plating constructs were stiffer than the Ogden construct in axial loading and torsion. Although no differences in loads to failure during torsion were noted, locked plating constructs exhibited catastrophic failure not observed with the Ogden construct.

Orthopedic surgical management of hip and knee involvement in patients with juvenile rheumatoid arthritis.

Juvenile rheumatoid arthritis is the most common arthritic disease of childhood and a leading cause of childhood disability, affecting an estimated 300,000 US children and adolescents aged < or =16 years. Approximately 10% to 30% of patients experience functional deficits resulting from both the articular and systemic manifestations of their disease, including leg length inequality and deformity, that are often more crippling than joint destruction. Surgical intervention to treat bone and soft-tissue deformity, leg length inequality, and joint destruction is indicated when medical therapy has failed. Synovectomy, soft-tissue release, osteotomy, and epiphysiodesis are used to treat deformity and early joint destruction. Arthroplasty remains the primary therapy for joint destruction, although it is fraught with complications specific to this young patient population.

The role of graft materials in suture augmentation for tendon repairs and reattachment.

Various biomaterials have been used to augment sutures for the repair and reattachment of tendons. This study examined four different graft materials in a simple and reproducible model using chicken Achilles tendons to determine the strength and mechanism of suture reinforcement of tendon repairs. The graft materials tested were Gore-Tex(R) Soft Tissue Patch, Graftjacket, bovine pericardium, and an experimental graft material from Xylos Corporation. Testing was performed in shear to simulate forces on a torn tendon repair and pull-off to simulate those on a tendon reattachment to bone. Compared to unaugmented suture, grafts increased suture fixation strength from 10% to 60% in shear and from 0% to 36% in pull-off with the bovine pericardium graft, providing significant improvement in both tests. In no cases (even unaugmented) did the suture pull directly through the tendon, but instead sliced along it, demonstrating that the interface between the suture and the tendon determines fixation strength. Grafts function by increasing the area, friction, and nature of this interface, not by acting as a barrier for suture pull-through.

The effect of locked distal screws in retrograde nailing of osteoporotic distal femur fractures: a laboratory study using cadaver femurs.

OBJECTIVES: To examine the effects of locked distal screws in retrograde nails used in unstable osteopenic distal femur fractures. DESIGN: Biomechanical testing of paired human cadaveric femurs. INTERVENTION: Seven matched pairs of embalmed, moderately osteopenic cadaver femurs were instrumented with 12-mm intramedullary nails in a statically locked, retrograde fashion. One femur of each pair had locked distal screws and the other femur had unlocked distal screws. A 2.5-cm gap of bone was cut nine centimeters from the distal condyles to simulate an unstable fracture. The locked distal screw nails were compared to unlocked distal screw nails for collapse of the fracture gap, medial-lateral and anterior-posterior translation of the nail within the fracture site, and fracture angulation. The femurs were axially loaded, cycled, and then loaded to failure. MAIN OUTCOME MEASURES: Motion at the fracture site with axial cyclic loading and site of failure when loaded to failure. RESULTS: After cycling, both locked distal screw and unlocked distal screw nails demonstrated several millimeters medial and anterior translation within the fracture site and approximately 1 mm collapse of the fracture gap. Although no statistically significant differences were found, the locked distal screw nails had less anterior and medial translation, angulation, and collapse of the fracture gap after cycling. Loads to failure were similar for both locked distal screw and unlocked distal screw nails. It was noted that proximal femur failure occurred at the level of the proximal screw hole in the nail at the subtrochanteric level in 7 (4 locked distal screws and 3 unlocked distal screw groups) of the 14 samples. Four other samples failed through the intertrochanteric region (2 locked distal screw and 2 unlocked distal screw groups) and the remainder within the distal fragment by fracture of the femur along the medial cortex. CONCLUSIONS: Although most differences in fixation stability were not significant, the locked distal screw nails exhibited less fracture collapse and anterior and medial translation of the nail at the fracture site than the unlocked distal screw nails. The degree of varus angulation after cyclic loading was also less for the locked distal screw nails. The length of the nail chosen should avoid having proximal locking screws distal to the lesser trochanter, thus averting proximal femur stress risers and fractures.

Suture versus screw fixation of displaced tibial eminence fractures: a biomechanical comparison.

PURPOSE: Classification and treatment of tibial eminence fractures are determined by the degree of fragment displacement. A variety of surgical procedures have been proposed to stabilize displaced fractures using both open and arthroscopic techniques. Two common fixation techniques involve use of cannulated screws and sutures tied over an anterior tibial bone bridge. We are unaware of any biomechanical studies that have compared the strength of various techniques of fixation. TYPE OF STUDY: Biomechanical study in a cadaveric model. METHODS: Seven matched pairs of fresh-frozen human cadaveric knees were stripped of all soft tissue except the anterior cruciate ligament (ACL). Simulated type III tibial eminence fractures were created using an osteotome. Fragments of each matched pair were randomized to fixation with either a single 4-mm cannulated cancellous screw with a washer or an arthroscopic suture technique using 3 No. 2 Fiberwire sutures (Arthrex, Naples, FL) passed through the tibial base of the ACL and tied over bone tunnels on the anterior tibial cortex. Specimens were then loaded with a constant load rate of 20 mm/min, and load-deformation curves were generated. The ultimate strength and stiffness were computed for each curve. The failure mode for each test was observed. A paired 2-tailed t test was used to determine the statistically significant difference between the two methods. RESULTS: Specimens fixed with Fiberwire had a mean ultimate strength of 319 N with a standard deviation of 125 N. Those fixed with cannulated screws had a mean ultimate strength of 125 N with a standard deviation of 74 N. This difference was statistically significant (P = .0038). There was no significant difference between the mean stiffness of Fiberwire constructs (63 N; SD, 50 N) and the mean stiffness of the cannulated screw constructs (20 N; SD, 32 N). The failure modes of the Fiberwire constructs included 1 ACL failure, 3 failures of suture cutting through the anterior tibial cortex, and 3 of suture cutting through the tibial eminence fragment. The single mode of failure for the cannulated screw constructs was screw pullout of cancellous bone. CONCLUSIONS: The initial ultimate strength of Fiberwire fixation of tibial eminence fractures in these specimens was significantly stronger than that of cannulated screw fixation. CLINICAL RELEVANCE: It appears that Fiberwire fixation of eminence fractures provides biomechanical advantages over cannulated screw fixation and may influence the type of treatment one chooses for patients with tibial eminence fractures.

Effects of preheating of hip prostheses on the stem-cement interface.

BACKGROUND: Debonding of the cement from metal implants has been implicated in the loosening of cemented total hip prostheses. Strengthening of the stem-cement interface has been suggested as a way to prevent loosening of the component. Previously, it was reported that preheating the stem to 44 degrees C reduced the porosity of the cement at the stem-cement interface. The purpose of this study was to determine the effect of stem preheating on the characteristics of the stem-cement interface. METHODS: The effects of stem preheating, at temperatures of 37 degrees C, 44 degrees C, and 50 degrees C, on the stem-cement interface were studied in a test model and a preparation that closely simulated the clinical situation. Static interface strength was determined initially and after the stems had been kept in isotonic saline solution at 37 degrees C for two weeks. Fatigue lifetimes were measured, and the nature and extent of porosity at the interface were quantified. RESULTS: Stem preheating had significant effects on the stem-cement interface. Stems preheated to 37 degrees C had greater interface shear strength than stems at room temperature both initially (53% greater strength) and after simulated aging (155% greater strength). Fatigue lifetimes were also improved, and there was a >99% decrease in interface porosity. The setting time of the cement decreased 12%, and the maximum temperature at the cement-bone interface increased 6 degrees C. Similar effects were found after preheating to 44 degrees C and 50 degrees C. CONCLUSIONS: Stem preheating had significant effects on the stem-cement interface, with significant improvements in the shear strength and cement porosity of the interface. Also, polymerization temperatures at the cement-bone interface increased. The possible biological effects of these increased interface temperatures at the cement-bone interface require further study.

Bone response to a novel highly porous surface in a canine implantable chamber.

Long-term survival of uncemented hip components is dependent upon successful biological fixation. This study examined a new prosthetic surface treatment consisting of a highly porous open structure of commercially pure titanium, Tritanium Dimensionalized Metal; its overall porosity is approximately 65-70%. With the use of an implantable chamber in dogs, the effects of this treatment on bone ingrowth and strength of attachment were compared to both titanium (overall porosity of 30-35%) and cobalt chrome beads (overall porosity of 35-40%), with and without hydroxyapatite coating. At 6 and 12 weeks, chambers were explanted and specimens underwent high-resolution radiographic imaging and mechanical testing. At 12 weeks, Tritanium surfaces had greater bone penetration and tensile strength than remaining surface types. Over 40% of the Tritanium specimens had a tensile strength greater than 500 N, exceeding the testing capability of the servohydraulic equipment. The highly porous Tritanium surfaces allow for a far greater amount of bone ingrowth than beaded surfaces, and may create a geometry that enhances mechanical strength. Tritanium Dimensionalized Metal surface treatment may result in a clinically valuable implant fixation surface to induce rapid ingrowth and a strong bone-implant interface, contributing to increased implant survivorship.

Loading of the acetabulum by polyethylene and all-ceramic inserts in metal-backed acetabular cups.

Both static and dynamic loads were applied to metal-backed acetabular cups with ceramic or polyethylene inserts and the resulting load transmissions at a simulated bone interface were determined. Perfect fit and under-sized and over-sized cavities were prepared in simulated bone substrates, lined with Fuji pressure sensitive film, and acetabular cups inserted with physiological loads. The magnitude and location of contact forces between the cup and bone were measured. These cups were then subjected to a controlled impact load and the intensity and frequency of the loads transmitted to the substrate were determined. The results suggest that a polyethylene backing for ceramics is not necessary as there were no major differences in the static and dynamic stresses transmitted to the cup-bone interface with all polyethylene or ceramic inserts.

Interobserver reliability and intraobserver reproducibility in suprascapular notch typing.

The size and shape of the suprascapular notch may be a factor in suprascapular nerve entrapment. The suprascapular notches of 623 scapulae were digitally photographed and used to determine notch type and area. Three researchers used to different classification systems for suprascapular notch typing. These systems were compared for interobserver reliability and intraobserver reproducibility using the kappa test. The mean kappa value for the classification used by Rengachary and colleagues was 0.468 and for the classification used by Ticker and associates was 0.531 for the inferior border of the notch and 0.736 for the superior border of the notch. The classification system used by Ticker and associates was more reliable and reproducible and produced both a superior and an inferior classification, making it possibly more clinically relevant than the classification system used by Rengachary and colleagues.

A new test for bipolar prosthesis disassociation.

Because of several failures by dissociation of a redesigned bipolar prosthesis, a new, dynamic test was developed. This dynamic cam-out test represents a closer simulation of one possible clinical mechanism of bipolar disassociation. The results of dynamic testing are affected by the bipolar design, particularly the locking mechanism that was the problem with the redesigned prosthesis.

Integrity of the stem-cement interface in THA: effects of stem surface finish and cement porosity.

No consensus exists for the optimal surface finish on cemented total hip prosthesis stems. The purpose of this study was to determine the effects of stem finish and interfacial cement porosity on the integrity of the stem-cement interface. Simulated stems made of Co-Cr, having polished or matte surfaces, at room temperature or heated to 37 degrees C, were cemented into Sawbones simulated femurs. Push out testing of the stem-cement interface was performed immediately after cement polymerization and after two aging periods in 37 degrees C saline or 37 degrees C air, and the extent of interfacial porosity at the stem-cement interface was determined. Polished stems exhibited an average 60% greater interfacial strength than that of matte stems initially and up to 240% after aging treatments. Cement porosity at the stem-cement interface and incomplete cement interdigitation into surface asperities on matte stems likely allowed saline penetration into the stem-cement interface during wet aging, resulting in a rapid decrease of shear strength. Stem preheating to 37 degrees C virtually eliminated interfacial pores and resulted in greater shear strengths regardless of surface finish. Polished stem surfaces with stem preheating provided the best interfacial shear strength and sealing ability against saline penetration into the stem-cement interface and could result in increased longevity of stem fixation.

The modified racking hitch(MRH) knot: a new sliding knot for arthroscopic surgery.

Sliding knots are commonly used in arthroscopic surgery and can be problematic in terms of strength and loop security. A new sliding knot for arthroscopic surgery, a modified racking hitch (MRH) knot, is described. The knot is essentially a modified cow's hitch knot that includes two self-locking loops. By tensioning the loop strands, a snug knot is created without backward sliding and provides excellent knot and loop security. In laboratory testing, the MRH knot provided comparable strength and security to other sliding knots. Experience gained from approximately 5 years of clinical use has added to the promise of the useful application of the MRH knot in arthroscopy.

A biomechanical comparison of two volar locked plates in a dorsally unstable distal radius fracture model.

BACKGROUND: This study compares the biomechanical stability of two volar locked plate systems for fixation of unstable, extra-articular distal radius fractures. METHODS: In six matched pairs of fresh frozen cadaveric specimens, a simulated unstable, extra-articular distal radius fracture was created. The fractures were stabilized with one of two types of volar locked plates. Specimens were axially loaded at five different positions: central, volar, dorsal, radial, and ulnar. Initial (precyclic loading) stiffness of each locked plate system was calculated. Each specimen was then loaded for 5,000 cycles with an 80 N central load. Finally, specimens were axially loaded at the same five positions to calculate the postcyclic loading stiffness of each volar locked plate system. Main outcome measurements were precyclic loading stiffness, postcyclic loading stiffness, maintenance of stiffness after cyclic loading, and amount of fracture displacement between the two volar locked plate systems. RESULTS: There were no differences in maintenance of stiffness and fracture displacement following cyclical loading between the two volar plate systems. After cyclic loading, the distal volar radius (DVR) locked plate was significantly stiffer than the Synthes volar locked plate in volar loading only (p < 0.01). CONCLUSION: Materials properties and design differences between these systems did not provide enough biomechanical difference to support use of either implant over the other. With this in vitro model, both implants provided adequate stability to resist physiologic loads expected during therapy in the initial postoperative period.

Stress risers between two ipsilateral intramedullary stems: a finite-element and biomechanical analysis.

Periprosthetic fracture after total joint replacement predominantly occurs at the stem tip. In this study, the effects of gap size, stem stability and cortical thickness between two press-fit, ipsilateral intramedullary stems on the tensile stresses created in the femur were investigated using finite-element models. The findings were confirmed with strain-gauge tests using a composite Sawbone femur. Gap size did not affect the level of stress on the femur. Cortical thickness had an important effect on stress distribution: peak stresses increased as bone cortical thickness decreased. Irrespective of gap size, the tips of loose stems acted as stress risers particularly with thinner cortices; the tips of well-fixed stems, however, did not.

Fixation of tendon grafts for collateral ligament reconstructions: a cadaveric biomechanical study.

PURPOSE: To compare the biomechanical properties of 4 methods of fixation of tendon grafts to bone as used for ligament reconstructions. METHODS: Thirty-two metacarpals were harvested from fresh-frozen cadavers and stripped of soft tissue. Flexor tendons were harvested from the same cadavers and cut into 2-mm-wide strips. Each tendon was fixed to a metacarpal head at the site of origin of a collateral ligament. Four different methods of fixation were tested. In group 1 the tendon was fixed to the bone with a 4.0-mm Arthrex bio-tenodesis interference screw (Arthrex, Inc., Naples, FL). In group 2 the tendon was passed through a bone tunnel and fixed with a 3.2-mm mini-Acutrak screw (Acumed, LLC, Hillsboro, OR) that was inserted in interference mode. In group 3 the tendon was passed through a bone tunnel and fixed with sutures tied over a polyethylene button. In group 4 the tendon was fixed with a mini-Mitek bone suture anchor (Mitek Worldwide, Norwood, MA). All specimens were clamped into a linear loading machine and loaded until failure. Statistical analysis was performed by 1-way analysis of variance testing. RESULTS: The differences in maximal tensile strength and stiffness were statistically significant when comparing any 2 groups. The Arthrex biotenodesis interference screw was the strongest and stiffest fixation method, followed by the Acutrak screw inserted in interference mode. Next was the suture tied over a button method. The mini-Mitek bone suture anchor was the weakest. CONCLUSIONS: Interference screw fixation of tendons to bone has statistically significant higher pullout strength and stiffness than 2 other commonly used fixation methods. The use of interference screws for fixation of tendon grafts to bone for hand ligament reconstructions is a promising new surgical technique.

Thumb metacarpophalangeal ulnar collateral ligament injuries: a biomechanical simulation study of four static reconstructions.

PURPOSE: To determine the optimal tunnel placement positions for tendon graft reconstruction of chronic thumb metacarpophalangeal (MCP) ulnar collateral ligament injuries that would stabilize the joint while maintaining motion. METHODS: Four commonly used tunnel placement methods were simulated on a cadaveric model using fresh-frozen thumbs and a suture/screw construct. The methods were as follows: (1) triangular configuration with apex proximal, (2) triangular configuration with apex distal, (3) cruciate configuration, and (4) parallel configuration. Stability was tested by valgus loading at 0 degrees and 30 degrees , and range of motion was tested by loading the thumb tendons. Statistical analysis was performed by 1-way analysis of variance testing. RESULTS: Valgus load stability testing at 0 degrees and 30 degrees showed that all 4 reconstruction methods stabilized the MCP joint compared with the fully sectioned state. The amount of stability achieved was not significantly different between the 4 methods. Only the reconstruction method, however, with a triangular configuration with the apex proximal restored flexion/extension range of motion not significantly different from the intact state. The other 3 methods resulted in significantly decreased range of motion. CONCLUSIONS: The reconstruction tunnel positioning of triangular configuration with apex proximal stabilizes the thumb MCP joint while maintaining flexion/extension range of motion. We recommend this configuration for chronic MCP joint injuries in which the native ulnar collateral ligament is inadequate and tendon graft reconstruction is performed.

Comparison of a sliding hip screw with a trochanteric lateral support plate to an intramedullary hip screw for fixation of unstable intertrochanteric hip fractures: a cadaver study.

BACKGROUND: The lateral trochanteric support plate (LSP) was developed to prevent excessive sliding of unstable intertrochanteric femur fractures fixed with a sliding hip screw (SHS). This study compared the fracture stability and screw sliding characteristics of unstable intertrochanteric femur fractures fixed with either an SHS and LSP or an Intramedullary Hip Screw (IMHS). METHODS: Six matched pairs of cadaveric human femurs with simulated, unstable intertrochanteric femur fractures were stabilized with either an IMHS or a 135-degree SHS with an attached LSP. Inferior and lateral head displacements and lag screw sliding distances were measured for applied static loads of 750 N, before and after cycling. RESULTS: Four-part unstable intertrochanteric femur fractures showed comparable screw sliding characteristics and stability whether instrumented with an SHS and LSP or an IMHS. CONCLUSION: A sliding hip screw with an attached lateral support plate provides stability and ability to resist medial displacement of the femoral shaft similar to that seen with the IMHS.

Comparison of the LISS and a retrograde-inserted supracondylar intramedullary nail for fixation of a periprosthetic distal femur fracture proximal to a total knee arthroplasty.

Simulated supracondylar fractures were created proximal to posterior cruciate ligament-retaining total knee arthroplasty components in paired human cadaver femora and stabilized with either a retrograde-inserted locked supracondylar nail or the Less Invasive Stabilization System (LISS; Synthes USA, Paoli, PA). Loads were applied to create bending and torsional moments on the simulated fracture stabilized with either no gap or a 10-mm gap. The LISS exhibited less torsional stability with anterior (P<.001) and posterior loads (P<.01). When varus loads were applied to 10-mm-gap specimens, the specimens stabilized with a retrograde nail had an 83% reduction in fracture displacement (P<.001) and 80% less medial translation of the distal fragment (P<.001). The samples stabilized with the LISS had a 93% reduction in fracture gap displacement when a valgus load was applied with a 10-mm gap (P<.001). Overall, these results suggest that the retrograde-inserted nail may provide greater stability for the management of periprosthetic supracondylar femur fractures in patients with a posterior cruciate ligament-retaining femoral total knee arthroplasty component.