Unicortical--bicortical behavior of a hybrid bone screw.

Stoia, Dan Ioan ; Toth-Tascau, Mirela

1. INTRODUCTION

Bone is a complex, highly organized and specialized connective tissue. The vertebra consists of a vertebral body anteriorly resembling a tubular bone with a central portion surrounded by a thin cortical shell. The posterior vertebral arch consists of the pedicles with dense cortices and relatively little intervening spongy bone and laminae which are flattened bones with also relatively dense cortex and less dense cancellous bone (Carter, 2002), (Tan, 2003).

Basically, there are two types of bone screws designed to be assembled with cortical or cancellous bone structures. Taking into account that the vertebral bone is a composite structure composed by a cancellous core and a cortical shell, the screw must proper threading the both bone structures (Butscher, 2003).

The bone screws are available in a variety of designs and materials. For instance, the treads may be vee, square or buttress shaped and the major and minor diameters may be straight or tapered. The helix pitch plays also an important role. These features affect the pull-out strength, the stiffness, and the holding power of the screw. It is assumed that a well-fixed screw will help maintain a surgical reduction until fusion occurs (Dawson, 2003).

In the framework of the project Researches Regarding the Improvement of the Modeling and Manufacturing Techniques for the Human Spine Implants, were designed, developed and tested three types of screws according to the bone structure: one screw type for cancellous vertebral bone, a second screw for cortical vertebral bone and a third screw, designed as a combination of the first two, named hybrid. All of the screws are destined to fix a plate implant in the cervical bone structure. This paper will present the torque function of angular displacement during the threading/screwing tests of the hybrid screw.

Due to the self-threading requirements of the hybrid screw, it is necessary to measure the torque needed to be applied on the screw (for a certain axial loading value), in order to realize the threading and screwing in the composite bone structure. The tests were performed for two cases: unicortical and bicortical threading (Stoia, 2008).

2. TESTING EQUIPMENTS AND METHODS

The tests were realized in the frame of CIDUCOS Testing Laboratory, accreditated by RENAR--Romanian Accreditation Association, on Vortex-i, testing equipment (Fig. 1.). The testing conditions:

* Axial load on the screw P=35 N;

* Angular velocity 2.5 rot/min;

* Maximum torque 1Nm;

* Angular displacement 4500 deg--equivalent of 12.5 rotations.

In order to measure the unicortical and bicortical screw behaviour, five cadeveric specimens of lumbar L4 vertebras were used. The vertebral body specimens were coded CV1, CV2 and CV3, while the vertebral pedicles were coded PV1 and PV2.

At the places of the screw insertions, in all the specimens, 2.5 mm holes were drilled. After that, these were fixed on the rotational table of the testing machine. The hybrid titanium screw was fixed in the grip of the torque cell, being aligned with the bone hole. The machine control was made by computer, using the dedicated Emperor software.

The type of the control program used for tests was run, who involves all the parameters presented before. The maximum angular displacement (9) was calculated using the following relation:

[theta] = 1 / p = 7.5 / 0.6 = 12.5 rot (1)

where: l represents the thread length of the hybrid screw and p, the pitch of the screw.

An important issue for the tests stability and validity is to have a good alignment between the bone holes and the screw.

3. RESULTS

The results express the mechanical behaviour of the hybrid screw when threading the two structures. The intermediary positions for threading the vertebral bodies and pedicles are presented in the Fig. 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Because of the large sampling rate of the measured values, only the tendency curves of the torques function of angular displacements will be presented.

In the Fig. 3. are presented the tendency curves of the torques function of angular displacements for the three vertebral specimens CV1, CV2 and CV3. The shape differences recorded for the three curves are generated by two factors: the first one is generated by the differences in the bone density between each specimen; the second factor is cause both by the differences in the angular starting position of the screw and the thickness of the bone layers (cortical and cancellous).

The first factor is responsible by the amplitude variation while the second generates a phase difference.

In the case of unicortical threading three characteristic intervals can be identified:

* Cortical threading--from 0 to 3.5 rot and a maximum torque of 0.11 Nm.

* Cancellous threading--from 3.5 to 11.5 rot with a relative constant torque. The spontaneous variations are determined by the variations in the bone density.

* Final screwing--from the 11.5 to 12.5 rot is characterized by the sudden increasing of the torque.

In the Fig. 4. is presented the bicortical threading behaviour of the PV1 and PV2 specimens. In the same manner as in the first case, the torques, function of angular displacements are presented..

The threading and screwing characteristic intervals for bicortical structures cases are:

* First cortical threading--form 0 to 3.5 rot--corresponding to the penetration of the first bone layer.

* Cancellous threading--from 3.5 to 6.6 rot--corresponds to the second bone layer and is characterized by the decreasing of the torque due to the reduced density of the layer.

* Second cortical threading--from 6.6 to 10.8 rot--record a maximum torque value of 0.25 Nm; 2.5 higher than in the first cortical threading.

* Pierce through--form 10.8 to 12.5 is the interval where the torque decreases due to the pierce of the second cortical layer.

The maximum torque values recorded in all the testing cases are very much dependent on the cortical layer thickness.

4. CONCLUSIONS

The performed tests prove the hybrid screw capacity to self-threading in both structures: unicortical and bicortical bone.

The maximum measured torques in both cases for an axial force of 35 N, are 0.11Nm for the first cortical threading and 0.25mm for the second cortical threading. These values indicate the surgeon effort, required during the implantation procedure.

It must be mentioned that, when threading and screwing in the living bone, the maximum torque values can be different due to the viscoelasticity of the living bone toward cadaveric bone.

Also, the torque values required for self-threading of the hybrid screw are dependent by the following factors: anisotropy of the bone structure (Dong, 2004), the age and the sex of the human subjects.

One can say that the measurements reveal some reference values for the torque, in the case of self-threading in the unicortical and bicortical bone structure.

5. REFERENCES

Butscher, A.; Schneider, R.; Wahl, D.; Gasser, B.; Linke, B. (2002). Holding strength of conventional and locked plate screws, ACTA of Bioengineering and Biomechanics, Vol.4, No.1, pp.782-783, 83-7085-639-x, Wroclaw, Poland.

Carter, D.R. & al. (2002). Bone in Clinical Orthopedics, Thieme, 3-13-125721-0, Stuttgart-New York.

Dawson, J.M.; Boschert, P.; Macenski M.M. & Rand, N. (2003) . Clinical Relevance of Pull-out Strength Testing of Pedicle Screws, In: Spinal Implants: Are We Evaluating Them Appropriately?, Melkerson, M.N., Kirkpatrick, J.S., Griffith S.L.,(ASTM International), pp.68-77,0-8031-34630, Dallas, Texas.

Dong, X.N.; Guo X.E. (2004). The dependence of transversely isotropic elasticity of human femoral cortical bone on porosity, Journal of Biomechanics, Vol.37, No.8, pp.1281-1287, 0021-9290, Elsevier.

Stoia, D.I.; Researches Regarding the Improvement of the Modeling and Manufacturing Techniques for the Human Spine Implants, Type TD, Code 95, Contract GR98/11.06.2008, Financed by the Romanian National University Research Council.

Tan, J.S.; Kwon, B.K.; Samarasekera, D.; Dvorak, M.F.; Fisher, C.G. & Oxland, T.R. (2003). Vertebral Bone Density-A Critical Element in the Performance of Spinal Implants, In: Spinal Implants: Are We Evaluating Them Appropriately?, Melkerson, M.N., Kirkpatrick, J.S., Griffith S.L.,(ASTM International),68-77,0-8031-3463-0, Dallas, Texas.