The effect of shoulder prosthesis stem length on failure due to torsional loading. A biomechanical study in composite humeri.

Background: Shoulder arthroplasty is becoming increasingly common. With evolving implant designs, multiple humeral stem options exist for the surgeon to choose from. New stemless and short-stem systems are modular, remove less native bone stock, and better adapt to patient anatomy. It has been suggested that shorter stem implants may be protective against periprosthetic fracture; however, this has not been mechanistically evaluated. Therefore, this study aimed to biomechanically test synthetic humeri with long-stem, short-stem, and stemless arthroplasty components in a torsional manner to evaluate their response to loading and characterize failure. Methods: Twenty-four synthetic humeri were implanted with long stem, short stem, or stemless uncemented prosthesis, 8 in each group. Humeri were mounted in a custom testing jig with a morse taper interfacing with a mechanical testing system. After a 20N axial force, specimens were torsionally loaded to failure at 15 degrees/sec, with 50 Hz collection. Torque vs. rotation curves were generated for each specimen, and stiffness, yield, ultimate strength, and failure load were measured. ANOVA and post hoc pairwise comparisons were used to assess effect of stem type on mechanical test variable. The association of the stem type with fracture type was analyzed by a Fisher's Exact test. Statistical significance was set at P < .05. Results: During torsional loading, long-stem implants were significantly stiffer than short or stemless implants. The angle of implant yielding was similar across stem designs; however, stemless implants had a lower yield torque. This correlated with a decreased yield energy in stemless compared to short stems as well. Maximum torque and failure torque was also significantly higher in short-stem and long-stem implants compared to stemless. Discussion: Periprosthetic fractures in shoulder arthroplasty are a concern in low-energy trauma, and stem design likely plays a significant role in early implant-bone failure. Our results suggest stemless implants under torsional load fail at lower stress and are less stiff than stemmed implants. The failure mechanism of stemless implants through metaphyseal cancellous bone emphasizes the effect bone quality has on implant fixation. There is likely a balance of torsional stability to survive physiologic loads while minimizing diaphyseal stress and risk of diaphyseal periprosthetic fracture. This combined with revision and fixation options represent decisions the surgeon is faced with when performing shoulder arthroplasty.

A Biomechanical Comparison of Trochanteric Versus Piriformis Reconstruction Nails for Femoral Neck Fracture Prophylaxis.

OBJECTIVES: To compare piriformis fossa to greater trochanteric entry cephalomedullary implants in an evaluation of femoral neck load to failure when the device is used for femoral shaft fractures with prophylaxis of an associated femoral neck fracture. METHODS: Thirty fourth-generation synthetic femur models were separated into 5 groups: intact femora, entry sites alone at the piriformis fossa or greater trochanter, and piriformis fossa and greater trochanteric entry sites after the insertion of a cephalomedullary nail. Each model was mechanically loaded with a flat plate against the superior femoral head along the mechanical axis and load to failure was recorded. RESULTS: Mean load to failure was 5487 ± 376 N in the intact femur, 3126 ± 387 N in the piriformis fossa entry site group, 3772 ± 558 N in the piriformis entry nail, 5332 ± 292 N for the greater trochanteric entry site, and 5406 ± 801 N for the greater trochanteric nail group. Both piriformis groups were significantly lower compared with the intact group. Both greater trochanteric groups were similar to the intact group and were statistically higher than the piriformis groups. CONCLUSIONS: A piriformis fossa entry site with or without an intramedullary implant weakens the femoral neck in load to failure testing. A greater trochanteric entry yields a load to failure equivalent to that of an intact femoral neck. Instrumentation with a greater trochanteric cephalomedullary nail is significantly stronger than a piriformis fossa cephalomedullary nail during axial loading in a composite femur model.

Are piriformis reconstruction implants ideal for prophylactic femoral neck fixation?

OBJECTIVES: Prophylactic femoral neck fixation may be performed in the setting of geriatric diaphyseal femur fracture, pathologic or impending atypical femur fractures. Fixation constructs often utilize cephalomedullary implants with one or two proximal interlocking screws into the femoral head/neck. Variations in proximal femoral anatomy and implant design can interfere with the placement of two screws in the femoral head and neck. Our objective was to assess the strength of piriformis entry reconstruction implants with one versus two proximal interlock screws for prophylactic femoral neck fixation. METHODS: Thirty fourth generation synthetic femur models were separated into 5 groups. The control group was an intact femur, and the second group was an intact femur with an entry hole in the piriformis fossa. The remaining groups had an intramedullary nail placed with either 0, 1, or 2 screws placed into the femoral head and neck. Each femur was mechanically loaded along the mechanical axis through the femoral head. Load to failure and failure displacement were recorded. RESULTS: Mean load to failure was 5583 ± 543 N in the intact femur. Constructs with 2 screws had a significantly higher mean load to failure (3223 ± 474 N) compared to one screw constructs (2368 ± 280 N). All of the experimental groups remained significantly lower than the intact femur model (p < 0.05). CONCLUSION: Our results demonstrate that piriformis entry reconstruction implants have a significantly lower load to failure compared to an intact femur irrespective of screw construct. Further studies are needed to investigate this potential iatrogenic weakening.

The Effects of Interlocking a Universal Hip Cementless Stem on Implant Subsidence and Mechanical Properties of Cadaveric Canine Femora.

OBJECTIVE: To determine if an interlocking bolt would limit subsidence of the biological fixation universal hip (BFX(®)) femoral stem under cyclic loading and enhance construct stiffness, yield, and failure properties. STUDY DESIGN: Ex vivo biomechanical study. ANIMALS: Cadaveric canine femora (10 pairs). METHODS: Paired femora implanted with a traditional stem or an interlocking stem (constructs) were cyclically loaded at walk, trot, and gallop loads while implant and bone motions were captured using kinematic markers and high-speed video. Constructs were then loaded to failure to evaluate failure mechanical properties. RESULTS: Implant subsidence was greater (P = .037) for the traditional implant (4.19 mm) than the interlocking implant (0.78 mm) only after gallop cyclic loading, and cumulatively after walk, trot, and gallop cyclic loads (5.20 mm vs. 1.28 mm, P = .038). Yield and failure loads were greater (P = .029 and .002, respectively) for the interlocking stem construct (1155 N and 2337 N) than the traditional stem construct (816 N and 1405 N). Version angle change after cyclic loading was greater (P = .020) for the traditional implant (3.89 degrees) than for the interlocking implant (0.16 degrees), whereas stem varus displacement at failure was greater (P = .008) for the interlocking implant (1.5 degrees) than the traditional implant (0.17 degrees). CONCLUSION: Addition of a stabilizing bolt enhanced construct stability and limited subsidence of a BFX(®) femoral stem. Use of the interlocking implant may decrease postoperative subsidence. However, in vivo effects of the interlocking bolt on osseointegration, bone remodeling, and stress shielding are unknown.

Spectroscopic and mechanical evaluation of thin film commonly used for banding congenital portosystemic shunts in dogs.

OBJECTIVES: To (1) determine whether different types of thin film used to occlude congenital portosystemic shunts are cellophane, and (2) evaluate the influence of saline immersion and sterilization on the tensile properties of cellophane. STUDY DESIGN: Ex vivo spectroscopic evaluation and mechanical testing. SAMPLE POPULATION: Rectangular strips of thin film from 4 sources. METHODS: Samples were evaluated with Fourier Transform Infrared Spectroscopy and microscopy with a polarizing lens. Samples consistent with cellophane were divided into 5 sterilization groups: non-sterile, autoclave, gamma irradiation, hydrogen peroxide and ethylene oxide. Samples were tested while dry or after saline solution immersion. Tensile properties were compared using ANOVA, unpaired t-tests, Mann-Whitney U-tests and Fisher's exact tests. P < 0.05 was considered significant. RESULTS: One thin film was consistent with cellophane and it could be differentiated from the other thin films by visible striations. Cellophane was strongest when strips were oriented parallel with its fiber direction and saline immersion reduced its strength by 48% (P < .001). All sterilization methods except autoclave significantly weakened wet cellophane (ethylene oxide [P < .001], gamma irradiation [P < .001], and hydrogen peroxide [P < .001]). CONCLUSIONS: Thin film from most sources was not consistent with cellophane. Autoclave sterilization is the best way to preserve the strength of wet cellophane.