Internal fixation with a self-compressing plate and lag screw: improvements of the plate hole and screw design. 2. In vivo investigations

Non-osteotomy and osteotomy large animal fracture models in orthopedic trauma research

Large animal fracture models are important in the field of orthopedic trauma research. New implants are tested in animals before being implanted into humans. Large animals like sheep or swine often are more properly to simulate conditions in humans, e.g. biomechanical demands, compared to rodents. Cited articles mainly analyze shock or fracture healing. Both osteotomy and non-osteotomy fracture models have been used in the past. However, comparative studies are rare and clear recommendation when to use which model are missing. This review will summarize large animal fracture models putting special emphasis on non-osteotomy fracture models.

The mechanical behavior of locking compression plates compared with dynamic compression plates in a cadaver radius model

OBJECTIVE

The purpose of this cadaveric study was to compare the mechanical behavior of a locked compression plate, which uses threaded screw heads to create a fixed angle construct, with a dynamic compression plate construct in a cadaver radius model.

DESIGN

Mechanical study with cyclic testing and high-speed optical motion analysis.

SETTING

Biomechanics laboratory at an academic institution.

PATIENTS/PARTICIPANTS

Eighteen pairs of fresh-frozen human cadaver radii were divided into 3 groups of 6 to be tested as a group in each of the following force applications: anteroposterior (AP) bending, mediolateral bending, or torsion.

INTERVENTION

Each bone was osteotomized leaving a 5-mm fracture gap and then fixed with a plate. For each pair, 1 radius received a standard plate (limited-contact dynamic compression plates; LC-DCP), the contralateral radius was fixed with a locking compression plate (LCP), and specimens underwent cyclic loading. Normalized stiffness, average energy absorbed, and Newton-cycles to failure were calculated. In addition, a 3-dimensional, high-speed, infrared motion analysis system was used to evaluate motion at the fracture site.

MAIN OUTCOME MEASUREMENTS

Construct stiffness, fracture site motion, cycles to failure, and energy absorption. Repeated measures ANOVA were used to detect differences between groups with time.

RESULTS

In the torsion group, LCP specimens failed at 60% greater Newton-cycles than the LC-DCP (1473 vs. 918; P < 0.05). In the AP group, the LC-DCP absorbed significantly greater energy during 10,000 cycles compared with the LCP group (P < 0.05). The 2 constructs demonstrated different biomechanical behavior with time. As cycling progressed in the LC-DCP specimens under torsion testing, stiffness (measured at the actuator at the bone ends) did not change significantly; however, fracture motion (measured at the fracture surfaces) decreased significantly (P = 0.04). The LCP specimens did not display similar behavior.

CONCLUSIONS

Our findings indicated that LCP constructs may demonstrate subtle mechanical superiority compared with the LC-DCP. The LCP specimens had less energy absorption in the AP group and survived longer in the torsion group. Discordance of motion between measurement regions was observed only in the LC-DCP torsion group, and may have been caused by plate-bone slippage or bone-screw subcatastrophic failure. However, many other compared parameters were found to be similar, and the clinical significance of the few differences found between constructs mandates further investigation.

The Effect of Divergent Screw Placement on the Initial Strength of Plate-to-Bone Fixation

BACKGROUND

Numerous implants exist that allow screws to be placed at varying angles for lag fixation or to fix additional fragments. This study determined how placing screws at different angles affects fixation strength.

METHODS

Using a bone model, we first investigated the pullout strength of screws inserted at varying angles; then, we studied the strength of plate-bone constructs with end screws placed at divergent angles.

RESULTS

Varying the screw angle from 0 to 10 to 20 degrees progressively weakened the screw pullout resistance. No additional decrease was found in varying the angle further. In contrast, the strength of fixation of plate to bone was higher for constructs with screws placed at 20 or 30 degrees off of perpendicular when tested in gap-open bending and axial compression (all p < 0.05). No such differences were found in torsion.

CONCLUSION

The pullout strength with angled screws is reduced, but this does not translate into reduced strength of the bone-to-plate interface.

Influence of the number of cortices on the stiffness of plate fixation of diaphyseal fractures

OBJECTIVE

To determine the influence of the number of cortices of fixation on the stiffness of plate fixation of diaphyseal fractures.

DESIGN

Canine experimental study.

SETTING

Tertiary referral and teaching hospital in Toronto, Canada.

PARTICIPANTS

Paired radii from fourteen skeletally mature, cross-bred dogs.

MAIN OUTCOME MEASURE

One member of each pair of radii was tested intact as a control, and the other had a transverse osteotomy plated sequentially with five to ten cortices of fixation on either side of the simulated fracture. Dynamic compression plates and limited contact dynamic compression plates were used in two groups with seven paired radii each. Normalized torsional stiffness and four-point bending stiffness were determined in the elastic range for the control and each of the plated constructs in both groups, using a materials testing machine.

RESULTS

The authors found no significant difference between the stiffness of the dynamic compression plates and limited contact dynamic compression plates. With either plate of a given length, significantly increased torsional stiffness is achieved with end bicortical screws. For bending stability with the plate at right angles to the bending plane, even short plated constructs have a stiffness exceeding that of intact bone.

CONCLUSIONS

For a transverse osteotomy with no fracture interdigitation, the bending rigidity with the plate at right angles to the bending plane is greater than the original stiffness of the bone for all constructs tested, with the exception of the limited contact dynamic compression plate with five cortices of fixation. The torsional rigidity of fixation only approaches the original rigidity of the bone for ten cortices of fixation with the dynamic compression plate and the limited contact dynamic compression plate.

Strength recovery in fractured sheep tibia treated with a plate or an internal fixator: an experimental study with a two-year follow-up

OBJECTIVES

Comparison of fracture healing with two different implants: a conventional Dynamic Compression Plate (DCP) and a new internal Point Contact Fixator (PC-Fix).

DESIGN

Randomized, prospective study in experimental animals. Observation times: 12, 24, 48 and 96 weeks, with six sheep per group.

SETTING

Following surgery, animals were kept with unrestricted weight-bearing in individual stalls for 12 weeks, thereafter in groups.

ANIMALS

56 adult Swiss mountain sheep.

INTERVENTION

A standardized oblique fracture of the sheep tibia was reduced and compressed by a lag screw and "neutralized" with one of the implants.

MAIN OUTCOME MEASUREMENTS

Standard radiographs were used for callus size measurements. After sacrifice the implant was removed and both the treated bone and the contralateral bone were tested for static strength in bending with the plate side under tension. Broken bones were processed for histological evaluation.

RESULTS

In the DCP group all six bones failed through the original fracture at 12 weeks. At 24 and 48 weeks two out of six, at 96 weeks one out of six bones failed through the original fracture, others through one of the screw holes. In the PC-Fix group there were no failures through the original fracture with a single exception at 96 weeks. The strength values in the PC-Fix groups of 12 and 96 weeks were significantly higher then in the corresponding DCP groups.

CONCLUSIONS

Healing of simple diaphysial fractures treated by PC-Fix was superior to that achieved by conventional plating. The histological evaluation suggested that the observed differences can be accounted for by the absence of implant-related cortical necrosis and by the circumferentially uninterrupted (if smaller) callus in the PC-Fix group.