Scott J. Hazelwood

A Mechanistic Model for Internal Bone Remodeling Exhibits Different Dynamic Responses in Disuse and Overload

Bone is a dynamic tissue which, through the process of bone remodeling in the mature skeleton, renews itself during normal function and adapts to mechanical loads. It is, therefore, important to understand the effect of remodeling on the mechanical function of bone, as well as the effect of the inherent time lag in the remodeling process. In this study, we develop a constitutive model for bone remodeling which includes a number of relevant mechanical and biological processes and use this model to address differences in the remodeling behavior as a volume element of bone is placed in disuse or overload. The remodeling parameters exhibited damped oscillatory behavior as the element was placed in disuse, with the amplitude of the oscillations increasing as the severity of disuse increased. In overload situations, the remodeling parameters exhibited critically sensitive behavior for loads beyond a threshold value. These results bear some correspondence to experimental findings, suggesting that the model may be useful when examining the importance of transient responses for bone in disuse, and for investigating the role fatigue damage removal plays in preventing or causing stress fractures. In addition, the constitutive algorithm is currently being employed in finite element simulations of bone adaptation to predict important features of the internal structure of the normal femur, as well as to study bone diseases and their treatment.

Development of an atrophic nonunion model and comparison to a closed healing fracture in rat femur

Although most fractures heal, some fail to heal and become nonunions. Many animal models have been developed to study problems of fracture healing. The majority of nonunion models have involved segmental bone defects, but this may not adequately represent the biologic condition in which nonunions clinically develop. The objective of the present study is to develop a nonunion model that better simulates the clinical situation in which there is soft tissue damage including periosteal disruption and to compare this model to a standard closed fracture model utilizing identical fracture stabilization, providing a similar mechanical environment. A total of 96 three month old Long Evans rats were utilized. A 1.25 mm diameter K‐wire was inserted into the femur in a retrograde fashion, and a mid‐diaphyseal closed transverse fracture was created using a standard three‐point bending device. To create a nonunion, 48 of the rats received additional surgery to the fractured femur. The fracture site was exposed and 2 mm of the periosteum was cauterized on each side of the fracture. Fracture healing was evaluated with serial radiographs every two weeks. Animals were maintained for intervals of two, four, six or eight weeks after surgery. Specimens from each time interval were subjected to biomechanical and histological evaluation. None of the cauterized fractures healed throughout the eight weeks experimental duration. The radiographical appearance of nonunion models was atrophic. This investigation showed pronounced differences between the experimental nonunions and standard closed fractures both histologically and biomechanically. In conclusion, we have developed a reproducible atrophic nonunion model in the rat femur that simulates the clinical condition in which there is periosteal disruption but no bone defect.

Osteon pullout in the equine third metacarpal bone: effects of ex vivo fatigue

An important concept in bone mechanics is that osteons influence mechanical properties in several ways, including contributing to toughness and fatigue strength by debonding from the interstitial matrix so as to „bridge”︁ developing cracks. Observations of „pulled out„ osteons on fracture surfaces are thought to be indicative of such behavior. We tested the hypothesis that osteon pullout varies with mode of loading (fatigue vs. monotonic), cortical region, elastic modulus, and fatigue life. Mid‐diaphseal beams from the dorsal, medial, and lateral regions of the equine third metacarpal bone were fractured in four point bending by monotonic loading to failure under deflection control, with or without 105 cycles of previous fatigue loading producing 5000 microstrain (15–20% of the expected failure strain) on the first cycle; or sinusoidal fatigue loading to failure, under load or deflection control, with the initial cycle producing 10,000 microstrain (30–40% of the expected failure strain). Using scanning electron microscopy, percent fracture surface area exhibiting osteon pullout (%OP.Ar) was measured. Monotonically loaded specimens and the compression side of fatigue fracture surfaces exhibited no osteon pullout. In load‐controlled fatigue, pullout was present on the tension side of fracture surfaces, was regionally dependent (occurring to a greater amount dorsally), and was correlated negatively with elastic modulus and positively with fatigue life. Regional variation in %OP.Ar was also significant for the pooled (load and deflection controlled) fatigue specimens. %OP.Ar was nearly significantly greater in deflection controlled fatigue specimens than in load‐controlled specimens (p < 0.059). The data suggest that tensile fatigue loading of cortical bone eventually introduces damage that results in osteonal debonding and pullout, which is also associated with increased fatigue life via mechanisms that are not yet clear.

Locked Plate Fixation of Osteoporotic Humeral Shaft Fractures: Are Two Locking Screws Per Segment Enough?

Objective: The purpose of this study was to compare the biomechanical behavior of using two versus three locking screws per bone segment in a cadaveric humerus fracture gap model. Methods: Six matched pairs of elderly osteoporotic fresh-frozen human cadaveric humerii were used. An eight-hole locking compression plate was placed posteriorly on the humeral shaft and secured with either four or six bicortical locking screws. A 5-mm middiaphyseal gap osteotomy was created to simulate a comminuted fracture without bony contact. Specimens were tested in offset axial compression, four-point anteroposterior bending, four-point medial-lateral bending, and torsion. After the initial testing in each of these modalities, the constructs were cyclically loaded in torsion and again tested in the four loading modalities. Lastly, the fixation constructs were then tested to failure in torsion. Results: There were no significant differences in stiffness between the group fixed with two screws per segment and the group fixed with three screws per segment. The peak torque to failure was higher in the four-screw construct compared with the six-screw construct. The mean torque to failure was 23.5 ± 3.7 Nm in the construct with two locking screws per segment compared with 20.4 ± 2.8 Nm in the construct with three locking screws per segment (P = 0.030). Conclusions: The addition of a third screw in the locked plate construct did not add to the mechanical stability in axial loading, bending, or torsion. In testing to failure, the addition of a third screw resulted in lower load to failure.

Long Stemmed Total Knee Arthroplasty With Interlocking Screws: A Computational Bone Adaptation Study

The ability of an interlocking screw fixation technique to minimize bone loss related to stress shielding in the tibia was investigated and compared to the abilities of cement and press‐fit fixation. Full bony ingrowth has been associated with greater stress shielding than partial ingrowth; therefore, the effect of intimate bonding of the stem to bone on subsequent bone loss was also studied. A damage‐ and disuse‐based remodeling theory was coupled with a two‐dimensional finite element model of the tibia to predict changes in bone remodeling following long stemmed total knee arthroplasty (TKA) for four different fixation techniques (cement, press‐fit, interlock with bony ingrowth, and interlock without bony ingrowth). Remodeling changes commenced with the model state variables—bone area fraction, mechanical stimulus, damage, and remodeling activity—at steady‐state values predicted by the intact tibia simulation. After TKA and irrespective of fixation technique, the model predicted elevated remodeling due to disuse, in which more bone was removed than replenished. In regions below the tibial tray and along the cortices, the interlocking stem with full bony ingrowth and the cemented stem caused the least amount of bone loss. An interlocking stem with a smooth, matted finish did not reduce the bone loss associated with interlocking fixation.

The Relationship Between Basic Multicellular Unit Activation and Origination in Cancellous Bone

Activation frequency is often used as a measure of basic multicellular unit (BMU) activity in cancellous bone. However, activation frequency expresses the rate of BMU appearance in a histologic slide and not the rate of origination, which is a more physiologic indicator of remodeling activity and is necessary for the development of BMU-level bone remodeling simulations. Using identical assumptions to those for calculating the activation frequency, it is shown that the origination frequency in cancellous bone is equal to the activation frequency divided by the total distance traveled by the BMU and its width.

The use of hinged external fixation to provide additional stabilization for fractures of the distal humerus

Objective: To assess improvements in fixation stability when a hinged unilateral external fixator is used to supplement compromised internal fixation for distal humerus fractures. Methods: Removing a 1-cm section of the distal humerus in cadaveric whole-arm specimens created a comminuted distal humerus fracture model (AO type 13-A3). Fixation was then performed using different constructs representing optimal, compromised, or supplemented internal fixation. Internal fixation consisted of either 2 reconstruction plates with 1, 2, or 3 (optimal) distal attachment screws, or crossing medial and lateral cortical screws. A hinged external fixator was applied in combination with compromised internal fixation. The stability of the different constructs was then evaluated using 3-point bending stiffness and distal fragment displacement measurements during flexion and extension testing. Results: Addition of the external fixator increased the stiffness of all constructs. Stiffness of the compromised reconstruction plate constructs with supplemented fixation was similar to or significantly greater than that of optimal internal fixation. Addition of the fixator to the reconstruction plates with 1 screw or the crossing screws produced displacements of the distal fragment that were similar to those of the compromised constructs alone. However, medial/lateral and anterior/posterior displacements of the distal fragment during flexion and extension of the elbow for supplemented fixation were found to be greater than those for optimal internal fixation. Conclusions: The use of a hinged external fixator for supplemental fixation of distal humerus fractures may be effective in cases where internal fixation is severely compromised, although displacements may increase above optimal fixation.

Fixation of the femoral condyles: a mechanical comparison of small and large fragment screw fixation.

BACKGROUND To compare the stability achieved using two 6.5-mm screws versus two or four 3.5-mm screws for the fixation of a unicondylar distal femur fracture. METHODS A fracture model was created in femoral synthetic composite bones to simulate a lateral femoral condyle fracture (AO/OTA 33-B1). Fixation was performed using three different types of screw constructs: 1) two 6.5-mm cancellous screws inserted using the lag technique, 2) two 3.5-mm cortical screws inserted using the lag technique, and 3) four 3.5-mm cortical screws, with two inserted using the lag technique and two as position screws. After reduction and fixation, the constructs were axially loaded in a material-testing machine. Main outcome measurements were the mean load required to displace the osteotomy site 1 and 2 mm as well as the mean stiffness of the different fixation methods. RESULTS The 6.5-mm construct required 56% more load to displace the osteotomy fragment 1 mm than the two 3.5-mm construct required (p < 0.0001), and 40% more load than the four 3.5-mm construct required (p < 0.0001). At loads that caused 2 mm of osteotomy displacement, these differences increased to 62% (p < 0.0001) and 48% (p < 0.0001), respectively. The mean loads needed to displace the osteotomy site were 28% higher for 1 mm of displacement (p = 0.003) and 27% higher for 2 mm of displacement (p = 0.03) for the four 3.5-mm screw construct compared with those needed for the two 3.5-mm group. The mean stiffness for the 6.5-mm group (1312.5 N/mm) was significantly higher than for the four 3.5-mm construct (784.2 N/mm; p < 0.0001) and the two 3.5-mm screw construct (409.4 N/mm; p < 0.0001). The difference in stiffness between the 3.5-mm groups was significant as well (p < 0.0001). CONCLUSION Stabilization of a unicondylar distal femur fracture with two 6.5-mm cancellous screws provides the most rigid and stable fixation. If small fragment screws are used, a minimum of four 3.5-mm cortical screws should be used to approximate the mechanical stability of two 6.5-mm screws.

Importance of the peridural membrane in percutaneous vertebroplasty.

Objective: The peridural membrane is a fibrous membrane that lies anterior to the posterior longitudinal ligament and attaches to its deep layer. It spans the width of the vertebral body and encircles the bony canal around the outside of the dura. The purpose of this study was to determine if the peridural membrane helps contain posterior cement leakage during percutaneous vertebroplasty. Methods: Compression fractures were experimentally created in cadaveric spines. The bodies were stabilized using bipedicular injection of cement, and injection was continued until cement was evident beyond the posterior border of the vertebral body. The vertebral segments were then dissected and the extravasated cement localized anatomically. Conclusions: All extravasated cement was constrained by the peridural membrane, and no direct contact of the cement with the dura was seen.

Total knee replacement with interlocking stems: a preliminary report.

Revision total knee replacements or primary total knee replacements with large amounts of bone loss frequently require long-stemmed prostheses to stabilize the components. The hypothesis of the current study was that long-stemmed prostheses can be fixed with interlocking screws, provide a successful outcome, avoid extensive stressshielding, and allow easy revision in the future. Ten patients with a minimum 2-year followup were studied. Outcomes were studied preoperatively and postoperatively using the Short Form-12 questionnaires, physical examinations, and radiographs. Complications were recorded. Ability to do activities of daily living, strenuous work, and sedentary work were recorded on a 10-point analog scale. Results indicated that 80% of stems well-fixed as shown on serial radiographs, functional outcome scores postoperatively improved significantly from preoperatively, pain was improved significantly, and positive bone remodeling without stress-shielding was seen in 80% of patients, but 20% had significant stress-shielding in the metaphysis. Complications included two periprosthetic fractures for which the patients did not require revision surgery, and one infection. Long-stemmed total knee components with interlocking fixation seem to be a reasonable alternative when large bone defects exist in the femur or tibia.