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

Article Outline

• Abstract
• Materials and methods
• Results
• Discussion
• Summary
• References
• Copyright

Abstract 

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. (The Journal of Foot & Ankle Surgery 42(1):15–20, 2003)

Keywords:  BONE-LOK, internal fixation, compression, cancellous screw, osteotomy, fracture

The cancellous bone screw is used commonly for the fixation of fractures, osteotomies, and arthrodesis of the lower and upper extremity, spine, and head and neck. This device works well in many situations. Certain circumstances, however, may lead to implant failure and morbidity. Loss of thread purchase and concomitant loss of fixation, compression, and stability of the fracture fragments may lead to malunion, delayed-union, or nonunion of the fracture, osteotomy, or arthrodesis ¹, ², ³.

Interfragmentary compression of 2 or more osseous fragments with a screw requires axial advancement of the screw. Simultaneous purchase of the head of the screw with the proximal bone fragment and the threads with the distal fragment must occur. As this process takes place, shear and frictional forces result from torque. Heat generation may cause osteonecrosis. Delayed healing and stripping may occur. In addition, threads may advance too far, and the cortex may crack. Some failures are related to poor bone quality, improper device selection, and inadequate technique ⁴, ⁵, ⁶.

A new device for cortical-cancellous fixation (Fig. 1) has been developed.

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• Fig. 1. 

  A 2.7-mm Triage Medical, Inc., HCA with BONE-LOK technology. Note the double-helix distal anchor and movable collar.

It was designed to improve interfragmentary compression while eliminating certain problems with the use of cancellous screws. Initial placement of this new device stabilizes the bone fragments, and secondary deployment of the device creates interfragmentary compression. The BONE-LOK (Triage Medical, Inc., Irvine, CA) ⁽⁷⁾ design enables interfragmentary compression from opposite sides of the bone-to-bone interface without applying torque to the device during the compression process. It provides fixation through the use of a unique distal thread design and a mobile proximal collar (U.S. and international patents issued and pending). The distal anchor is attached to a ratchet pin. The collar, which moves on the ratchet pin, makes each device adjustable over a 10-mm range of sizes. The double-helix thread is designed to advance quickly through cancellous bone while minimizing damage to the bone during the insertion process. Interfragmentary compression with the helical compression anchor (HCA) is not codependent on the advancement of the device further into the distal fragment, a feature that may eliminate far-side overhang, and the damage that this prominence may cause. The use of the HCA is similar to other cannulated systems and has a low-profile collar.

The purpose of this study was to establish objective, in vitro data for the evaluation and comparison of the 2.7-mm HCA with a larger-diameter 3.0-mm titanium cancellous screws (CS).

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Materials and methods 

Mechanical testing was designed to compare the 2.7-mm titanium HCA in length (range 14–24 mm) with the 3.0-mm short-thread, self-tapping, cannulated titanium CS in 24-mm length separately for pullout, 3-point bending, and compression strength. For pullout and compression testing, 7 lb/ft³ and 12 lb/ft³ rigid unicellular polyurethane foam (Pacific Research Laboratories, Vashon, WA) was used. The foam is supplied in 1-in (25.4 mm) thick sheets with foam rise direction in line with the thickness. These densities are within the range of cancellous bone. An 858 Mini Bionx test frame with 2,500 N load cell controlled by Teststar-SX version 4.0C with Testware software (MTS Systems Corporation, Eden Prairie, MN) was used for the pullout and bend testing. A PHM-100 Load Cell Indicator and THA-250-Q 250-lb load cell (Transducer Techniques, Temecula, CA) were used for the compression testing. Standard orthopedic instruments common to bone fixation procedures were used to install the 2.7-mm HCA and the 3.0-mm CS ⁽¹⁾. A sample size of 10 was used for each device for each test. All of these tests were performed at the Orthopedic Biomechanics Research Center, Children's Hospital, San Diego, California by unbiased technicians.

The pullout strength tests were performed using a polyurethane foam model to determine the maximum holding strength of the device. Similar models have been used to compare different types of orthopedic screws ⁽¹⁾ (Fig. 2).

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• Fig. 2. 

  Pullout testing apparatus pictured from above. A constant load was placed on both devices until failure occurred.

The 2.7-mm HCA and the 3.0-mm CS for comparison testing were each threaded into the foam a distance of 5 mm. The attachment of the devices and foam sheets to the test frame was performed by placing the device inserted in foam under a plate attached to the base of the test frame. The plate had a 19-mm diameter hole to pull the fastener through while supporting the foam block. A chuck was used to hold the head of the HCA or CS. The chuck was attached directly to the actuator in line with the load cell on the test frame. The devices were then pulled out of the foam using a constant displacement rate of 0.5 mm/s until failure. Failure was defined as a loss of load with increasing displacement. The maximum force obtained during loading was then recorded for each test.

The 3-point bending test was used to compare structural bending properties of the HCA and the CS. The 3-point bend fixture was set up with a 14-mm distance between the centers of the supports. The bending force was centered between the lower supports. A 1.0-mm diameter dowel pin was used for roller attachments on each support. The rollers were bonded to the supports. Markings were made on the supports to aid alignment of the fasteners with the loading axis of the actuator. The lower supports were attached to the base of the test frame and the upper support was attached to the actuator in line with the load cell (Fig. 3).

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• Fig. 3. 

  Three-point bending test of the 2.7-mm HCA. The 3.0-mm CS was tested with an identical setup.

Loading was applied at a constant rate of 2 mm/min until failure. Failure was defined as a rapid loss of load with increasing displacement. During the test, the load and displacement of the actuator was collected at 10 Hz.

Compression strength testing was performed to determine the compression strength capability of the HCA and CS during insertion into the different foam densities. The devices were installed using the appropriate insertion technique. A metal plate with a hole sized to the HCA or CS was placed between the load cell and HCA collar or the CS head. A screwdriver (Synthes, Paoli, PA) was used to apply a clockwise torque (without axial loading) to the CS (Fig. 4).

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• Fig. 4. 

  Setup for compression test of the 2.7-mm HCA. The adjustable collar was advanced to create compression until failure occurred. A similar setup was used for the 3.0-mm CS.

The test was completed when the CS stripped itself from the foam. A compression tool (Triage Medical, Inc.) was used to advance the collar of the HCA distally (without axial loading) over the ratchet pin to create compression. The test was complete when the HCA stripped itself from the foam. In each case, the load cell indicator was set to record the peak force generated across the compression load cell. Statistical analysis for each group was performed using the Student t test. A P value of .05 was set for statistical significance.

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Results 

The results of each test were recorded, studied for statistical significance, and are summarized in Tables 1 through 5.

Table 1. Foam block pullout strength for 12# foam

Table 2. Foam block pullout strength for 7# foam

Table 3. Three-point bending stiffness

Table 4. Compression strength testing for 12# foam

Table 5. Compression strength testing for 7# foam

The pullout testing results in 12# foam show that statistically greater force (nearly 2×) is required to shear the purchase of the HCA when compared with the CS of a slightly larger diameter. This difference is not statistically different in 7# foam, suggesting that holding ability may be foam-density dependent (Fig. 5).

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• Fig. 5. 

  Bar chart summarizing pullout strength test data. Pullout strength was greater for the 2.7-mm HCA in both foam densities, but not significantly greater in the #7 foam.

Three-point bending comparison of the 2.7-mm HCA with the CS showed that the HCA (988 N/mm) is approximately 17% stiffer than the CS (845 N/mm). These values were statistically significant (P < .001) (Fig. 6).

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• Fig. 6. 

  Bar chart summarizing bending test data. Greater load was required to deform the 2.7-mm HCA than the 3.0-mm CS.

These data represent an ability of the HCA to resist deformation and distraction/displacement of the repaired fragments when loads are applied to the HCA shaft.

Comparison of the compression strength of the 2 devices shows better compression of the 2.7-mm HCA over the 3.0-mm CS. Converse to the findings in pullout strength, the compression strength of both devices improves with decreasing foam density (Fig. 7).

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• Fig. 7. 

  Bar chart summarizing compression strength test data. The 2.7-mm HCA clearly outperforms the 3.0-mm CS in both the 7# and 12# foam.

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Discussion 

Data from this in vitro study shows greater pullout strength, 3-point bending stiffness, and compression strength of the 2.7-mm HCA when compared with the 3.0-mm CS in the synthetic foam model. Although the difference in pullout strength for the HCA in 7# foam is not statistically different than for the CS, the data suggests that the ability of the HCA to hold in lighter density foam compares favorably with the larger-diameter CS. This may represent an advantage because the HCA, by nature of design and size, does not disrupt as much bone as the CS in the process of insertion into the distal fragment in the context of this test model.

A clear distinction between the 2 devices becomes apparent in the compression tests in 12# and, more noticeably, 7# foam. The HCA shows over 4 times the compression strength in low-density foam, which may be a relevant and important observation when considering the relationship that lighter densities of foam have to decreased bone densities in vivo.

The results of these tests in synthetic foam do not represent the in vivo performance of the HCA and CS, but provide a controlled study for the comparison of these different devices. This study in synthetic foam does not include the effects that movement, vibration, and physiologic changes (that occur in bone during the healing process) have on internal fixation. Although considerable effort was made to design the studies to eliminate bias toward either device, it must be recognized that the 2 devices are very different in their design, and may, therefore, perform differently in alternate conditions.

The HCA can be applied to a number of applications in foot and ankle surgery, including distal and proximal metaphyseal osteotomies of the first metatarsal, intertarsal, and tarsal osteotomies and arthrodesis. Certain advantages of the device may include higher compression and pullout strength, and a distal anchor that does not advance while interfragmentary compression is applied. This latter feature may be useful especially for osteotomies positioned close to joints, as in the distal metaphyseal osteotomy used for hallux valgus correction. The HCA may be quicker to use because length determination does not require a separate step, and insertion does not require tapping. The adjustable nature of the HCA allows each device to cover a broad range of sizes, thereby reducing the need for large inventories of fixation devices to equip a screw fixation system. The HCA does, however, require the use of an additional instrument, the compression tool, which may be considered a disadvantage. Study in vivo is required to appreciate the potential of this device fully.

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Summary 

The adjustable 2.7-mm HCA is introduced. This device has a distal double helical anchor, and uses a ratchet pin and axially movable collar to create compression without the application of torque. The 2.7-mm HCA required greater loads than the 3.0-mm CS to achieve failure in all tests of pullout, bending, and compression in synthetic foam testing models. These results may correlate with greater stability between osseous fragments in the clinical model.

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References 

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