Fatigue life analysis of notched versus pristine spinal rods

Fatigue life analysis of notched versus pristine spinal rods

The Orthopaedic Innovation Centre Canada investigates the effect of spine rod gripping on fatigue strength through cyclic biomechanical testing.

Thoracic and lumbar back pain is a highly prevalent issue that most adults will suffer from at some point in their lives. Severe back pain can be caused by mechanical issues and soft-tissue injury that may require treatment through surgical intervention. Spinal fusion is a common remedy that immobilises the affected area of the spine.

General practice for thoracic and lumbar spinal fusion is to implant pedicle screws into neighbouring vertebrae and connect levels with spinal rods. Rods are bent to the desired curvature using a rod bender, then strategically positioned into each pedicle screw head, or tulip, using a specialised surgical instrument known as a rod gripper to hold the rod tightly. As a result, the rod gripper can create notches on the rod surface that are generally situated in the span between vertebrae and are exposed to significant bending loads. Unfortunately, instrument-generated notching on spine rods may increase the risk of hardware failure from cyclic (fatigue) loading and fusion failure.

The effect of notching on spine rod fatigue life has been tested by the engineers and technologists at the Orthopaedic Innovation Centre (Winnipeg, Canada). They have combined orthopaedic practice with biomechanical testing to determine the effect of instrument-generated notching on the fatigue endurance of two rod materials. Defining the detrimental effects of rod notching will aid in establishing the importance of preventing these notches intra-operatively, thus ensuring a more durable construct.

Titanium and cobalt-chromium-molybdenum (CoCrMo) alloy 5.5mm and 6.0mm diameter spine rods were tested in pristine (unnotched) and notched condition (Figure 1). Notches were simulated through a spine rod gripper applied by an orthopaedic surgeon. A single rod grip was carried out at maximum hand-grip force at the centre the rod which generated a set of four notches; two pairs opposite to each other (180 degrees around the circumference of the rod). The notches were initiated to represent worst case of four marks between each vertebral fusion level. Notch sensitivity was evaluated by comparing results to the fatigue life of rods in pristine condition.

Figure 1: Notched spinal rod showing a pair of instrument induced notches (top) versus a pristine spinal rod (bottom).

Biomechanical testing followed recognised standards for medical device testing of surgical fixation in the spine (ASTM F2193-18) and simulated physiological loading conditions on the lower back following spinal fusion. This followed a four-point bend cyclic test subject to 1.5 million cycles or until spine rod failure (Figure 2). Run-out was defined as 1.5 million cycles. Fatigue tests were carried out at maximum/minimum loads of -450N/-45N, -550N/-55N, and -650N/-65N for two samples per material group. Cyclic sinusoidal loads were applied at frequencies of 4 to 5Hz depending on rod stiffness. The number of cycles until failure was recorded to generate M-N curves for fatigue limit analysis (ASTM F2077). Rod stiffness (N/mm) was measured and fracture surfaces were examined.

Figure 2: Biomechanical test assembly and rod failure after four-point bend cyclic loading

It was hypothesised that notching would reduce fatigue life as a result of induced stress risers promoting early crack initiation sites. Furthermore, it was predicted that titanium alloy rods would show higher notch sensitivity than CoCrMo alloy rods based on known material properties.

Fatigue test results of rods in pristine condition showed marginally better endurance for the CoCrMo alloy rods than the titanium alloy rods at the applied loading conditions. Both the CoCrMo rods and 6.0mm titanium rods completed 1.5 million cycles up to -650N/-65N. However, fatigue life of the 5.5mm titanium rods was reduced to an average of 107,000 cycles when loaded to -650N/-65N. This result was expected as stress increases when cross-sectional area is reduced.

Cobalt-chrome-molybdenum spine rods proved to have higher fatigue resistance to notching than titanium rods. All notched CoCrMo rods survived to run-out in all three physiological loads tested compared against all of the notched titanium rods which failed within the first 185,000 cycles and 115,000 cycles for the 6.0mm and 5.5mm diameters, respectively. At the highest load tested (-650N/-65N), the notched titanium rods failed at an average of 62,000 cycles and 48,000 cycles for the 6.0mm and 5.5mm rods, respectively.

Our results demonstrate titanium spine rods to be notch sensitive when measured for fatigue endurance. Titanium rods were measured to have lower stiffness (6.0mm rod average = 113N/mm; 5.5 mm rod average = 83N/mm) than CoCrMo rods (6.0mm rod average = 272N/mm; 5.5mm rod average = 178N/mm). Longer fatigue life as measured in the CoCrMo rods may be explained by greater stiffness, as stiffness improves material resistance to applied loads. Material hardness may also explain higher fatigue resistance in the CoCrMo rods as notches were more difficult to create and therefore less prominent than on the softer titanium rods. Therefore, greater stress concentrations may have developed on titanium rods resulting in reduced fatigue life.

All failed rods fractured at the notch (Figure 3) which confirmed our hypothesis that instrument-induced notching created crack initiation sites and promoted early construct failure. This result demonstrates the importance that care should be taken when positioning titanium rods for spinal surgery as it may result in premature construct failure necessitating revision surgery.
In summary, this research has studied the effect of notching on the fatigue life of titanium and CoCrMo alloy spinal rods for thoracic and lumbar fusion surgery. Clinical loading conditions have been applied through biomechanical cyclic testing to measure the detrimental effects of using a spine rod gripper during rod positioning. This effect was shown to be significant for the titanium rods, however the material resistance of CoCrMo was greater and therefore did not show any reduction in fatigue life.

Figure 3: Crack initiation developed at notch resulting in rod fracture from cyclic loading.

While the study addresses clinical loads, the authors plan to perform additional fatigue testing at higher loads to investigate the fatigue life of CoCrMo rods and the limits of notching.

The Orthopaedic Innovation Centre continues to perform biomechanical testing on medical devices to improve product development and clinical outcomes. All collaborators on this topic would like to acknowledge the Alexander Gibson Fund at the University of Manitoba for funding this research. The researchers and engineers have no relevant industry disclosures.

 

References

  1. ASTM F2193-18 Standard Specifications and Test Methods for Components used in the Surgical Fixation of the Spinal Skeletal System
  2. ASTM F2077-14 Test Methods for Intervertebral Body Fusion Devices

 

Authors:

Sara Parashin is a biomedical engineer at the Orthopaedic Innovation Centre (Winnipeg, Canada) who dedicates her time to orthopaedic research studies, engineering testing, and explant retrieval analysis. Sara leads the biomechanical testing and clinical research departments related to the spine.

Brett Molchan is a biomechanical technologist at the Orthopaedic Innovation Centre (Winnipeg, Canada) who performs medical device testing, including wear test simulations and mechanical testing of implants. He ensures all testing is performed according to recognized standards and designs custom fixtures tailored to each test.

Kevin Stockwell is an MD and engineer in training with interest in biomedical research. He has conducted several research studies at the Orthopaedic Innovation Centre (Winnipeg, Canada) on the lifespan of arthroplasty and spine implants.

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