Improving structural behavior of a medical rod.

Bandrabur, Diana Lilia ; Bejan, Mihai ; Muntean, Angela Moroianu Corneliu 等

1. INTRODUCTION

After fracture reduction, osteo synthesis is the second aim in a bone fracture treatment. The shape and function of a human limb are restored after this approach. Metal implants, rods, brooches, screws or shells, used in osteo synthesis must have such physical and chemical properties which will allow reconstruction of the fractured bone (Gheorghiu & Cotet 1999). There are two important requirements for this. The first one is to ensure a robust structural bone--metal implants assemble and the second one is to be able to induce no human body rejecting reaction. Many times, in organism versus metal implant the last one reaches the failure point. The simplest approach in order to improve structural behavior of a medical rod is by finding the most favorable rod position related to bone fracture--figure 1. This allows us putting forward some simple recommendations for this particular medical rod studied and may be some patterns for other rod types (Hadar & Gheorghiu 2005); (Gheorghiu et al., 2000 a); (Gheorghiu et al., 2000 b).

2. FEM ANALYSIS OF A STRAIGHT MEDICAL ROD

Structural analysis was performed using finite element method. Side rod surface is 740 [mm.sup.2]. Material is chrom. Young modulus is E = 210000 MPa and Poisson coefficient v = 0.3.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

We set the limit stress at [[sigma].sub.axial lim] = 500 MPa for axial loading and [[sigma].sub.bending lim] = 250 MPa for bending.

There are two mechanisms producing the rod failure: fatigue and overloading. This study will consider the overloading case.

It is important to quantify the effects of relative positioning of medical rod related to bone fracture. Von Misses equivalent stresses [MPa] and result displacements [mm] reveals structural responses for all load cases.

2.1 Actual configuration

The most frequenly used configuration is the bone fracture at the middle point of the rod--fig. 1a.

Compression loading take into consideration distributed force on three holes. The 500 MPa limit stress is produced by a [F.sub.axial case A] = 1252 N resultant force--fig. 2a. The maximum resultant displacement for this load case is [[delta].sub.res max case A] = 0.02 mm. All important results are syntetic revealed in table 1.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The bending load is applied on half of the rod. The 250 MPa limit stress is produced by a [p.sub.bending case B] = 0.03131 MPa which correspond to a [F.sub.bending case B] = 11.6 N resultant force--fig. 2b. The maximum resultant displacement for this load case is [[delta].sub.res max case B] = 0.23 mm.

2.2 Alternative configurations

Alternative A configuration has the bone fracture between the third and the fourth holes and load on the biggest side surface of the medical rod. This will be considered the active part of the rod and the smaller side will be the fixed one--fig. 1b.

Compression loading take into consideration distributed force on four holes. The 500 MPa limit stress is produced by a [F.sub.axial case C] = 1815 N resultant force--fig. 3a. The maximum resultant displacement for this load case is [[delta].sub.res max case C] = 0-03 mm.

The bending load is applied on 410 [mm.sup.2]. The 250 MPa limit stress is produced by a [p.sub.bending case D] = 0.03111 MPa which correspond to a [F.sub.bending case D] = 12.76 N resultant force--fig. 3b. The maximum resultant displacement for this load case is [[delta].sub.res max case D] = 0.42 mm.

Alternative B configuration has the bone fracture between the forth and the fifth holes and the load on the smallest side surface of the medical rod. This is the active part of the rod and the bigger side is the fixed one--fig. 1c.

Compression loading is applied distributed on three holes. The 500 MPa limit stress is produced by a [F.sub.axial case E] = 917 N resultant force--fig. 4a. The maximum resultant displacement for this load case is [[delta].sub.res max case E] = 0.019 mm.

The bending load is applied on 330 [mm.sup.2]. The 250 MPa limit stress is produced by a [p.sub.bending case E] = 0.0607 MPa which correspond to a [F.sub.bending case E] = 20.02 N resultant force--fig. 4b. The maximum resultant displacement for this load case is [[delta].sub.res max case F] = 0.26 mm. Values of alternative configurations are normed regarding actual configuration--table 2.

3. CONCLUSIONS

Comparing A configuration with actual one the maximum equivalent axial force is 563 N bigger which reveals a 45 % increase of load carring capacity. For alternative B configuration 335 N smaller reveals a 26.7 % decrease. Bending the rod the A alternative configuration brings a 1.1 N bigger force which is a 10 % increase. More significant load carring capacity increase is for alternative B configuration where 8.42 N more represents 72.6 %. Results are consequences of structural design, load type and stress limits.

For this particular medical rod, both alternative configurations bring an increase in load carring capacity when bending is the applied load. When axial load is implied the alternative A configuration increases the load carring capacity while the alternative B configuration decreases it. For all medical rods the recomandation is to place the bone fracture between screw holes which are stress concentrators.

Further research can be orientated to create some general patterns for related types of medical rods (Hadar et al., 2004).

4. REFERENCES

Gheorghiu, H. & Cotet, C. (1999). Preliminary Selection of Biomaterials for Orthopaedic Implants, Technical Publishing House, Bucharest

Gheorghiu, H., Hadar, A., Jiga, G. & Cotet, C.E. (2000). Mechanical Behavior Study of Rod Implant For CyphoScoliosis Correction, The 8-th International Conference of Stress Measurement, Material Testing, pp. 526-531, 97331-1492-8 and 973-31-1494-4, June 2000, Constanta

Gheorghiu, H., Dumitrescu, E., Hadar, A. & Cotet, E. C. (2000). Preliminary Three-dimensional Model Regarding Improvement of Harrington Medical Devices Used for Some Spine Malformation Correction, The 8-th International Conference of Stress Measurement, pp. 512-517, 973-31-1492-8 and 973-31-1494-4, June 2000, Constanta

Hadar, A., Gheorghiu, H., Cotet, C. E. & Ciobanu, L. F (2004). 3d Model For a New Type of Implant in the Treatment of Cypho-Scoliosis, International Conference "Biomaterials & Medical Devices"--BiomMedD' 2004, p. 132, November 2004, Bucharest

Hadar, A. & Gheorghiu, H. (2005). An Optimized Solution for the Implant used in the Treatment of Cypho-Scoliosis, 22nd DANUBIA-ADRIA Symposium on Experimental Methods in Solid Mechanics, Terme, M., pp. 38-39, September 2005, Parma

Tab. 1. Absolute results

 Actual Alternative A Alternative B
Load type/result configuration configuration configuration

 Axial/maximum
equivalent force 1252 1815 917
 [N]

 Axial/maximum
 result 0.02 0.03 0.018
 displacement
 [mm]

 Bending/maxim
 um equivalent 11.6 12.76 20.02
 force [N]

 Bending/maxim
 um result 0.23 0.42 0.26
 displacement
 [mm]

Tab. 2. Normed results

Load type/result Actual Alternative Alternative
 A B

Axial/force 1 1.45 0.73
Axial/displacement 1 1.5 0.9
Bending/force 1 1.1 1.72
Bending/displacement 1 1.82 1.13