Implant testing protocols should be devetoped with the surgical goals in mind, making every attempt to model the in vıvo conditions accurately. The mechanical realities of biomechanical testing of spinal implants, can be summarized as:

1. No single biomechanical test fuiiy characterizes the ultimate surgical performance of an implant. İn fact, it can be argued that the biomechanical testing results reported to date on spinal implants suffer from many limita-tions.

2. No test, regardless of complexity, compieteiy characterizes the in vivo situation. İn most tests, loading tends to be simplified, the models used have been animals or ligamentous human cadaver material, and the num-bers and types of implants tested have been small.

3. Ali biomechanical models, vvhether in vitro or in vivo are just that, models. Even il in vivo tests are per-formed, one must stili ask the question, how well does the test model the clinical situation. Furthermore, the results may stili be difficult to generalize to overall population.

4. Spinal implants are no different than other mechanical device. They obey the basic laws of physics and en-gineering principals. For example, larger implants tend to be stiffer and stronger, stress concentrations increase a device's risk of fatique failure, implants utilizing more points of attachment to the spine tend to be stiffer and stronger, or the static strength of any method of fixation is no greater than the bone it is attached to.

5. Performance standarts of spinal implants are difficult t o define due to a lack of knovvledge about the in vivo situation. Hovvever, the relative performance of various implant designs to different tests has been found to be an effective method of judging performance.

6. The regulatory concerns of the US FDA do not necessarily follovv the concerns of surgeons, researchers, designers, or manufactures. For example, the FDA often requires test results relative to pre 1976 devices. Such testing is of little interest to surgeons who have abandoned the older devices. Hence, testing performed for regulatory considerations is often considered separately.

7. VVİthout appropriate statistical treatments, any conclusion is possible. Biomechanical testing must be sub-jected to the same rigors of the scientific method as any other field

8. From the surgeon's standpoint, the most relevant testing is that vvhich directly assess the surgical goals.

a) Can an implant impart corrective forces to a spine?

b) What is the stability of the device, both in terms of immediate and long term performance?

c) What effect will the implant have on fusion?

Keywords: Posterior implants, Biomechanical testing