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So much of what we write amounts to an argument for rigorous gatekeeping by trial courts. That’s true when we discuss Twombly/Iqbal. That’s true when we discuss Daubert. That’s also true when we discuss causation issues. “Causation” is a broad term that can include a couple of different things. For example, cases should be tossed if the conduct in question had no effect on the decision to use a product. Right now, we’re talking about classical medical causation — whether the product produced the alleged injury. We once took a Law and Economics class that offered a mathematical formula for causation. At the time it even seemed to make sense.

A good medical device causation decision recently came out the Western District of New York. In Donovan v. Centerpulse Spine-Tech Inc., 2010 U.S. Dist. LEXIS 31184 (WDNY March 30, 2010), the court cut through a lot of flotsam and jetsam to issue a summary judgment to the defendant based primarily on lack of medical causation. (The court also disposed of a failure to warn claim by applying the learned intermediary doctrine and concluding that the product insert furnished detailed information concerning the relevant risks.)

Plaintiff had undergone a lumbar laminectomy and spinal fusion surgery. The surgeon used defendant’s Silhouette Spinal Fixation (“SSF”) system, which involved screws and rods. Some time after surgery, plaintiff complained of numbness and pain. That’s not altogether unexpected, but the surgeon reviewed x-rays and saw that the right sacral screw had fractured. Another surgery removed the SSF system, though a portion of the screw had become embedded in the spine and had to be left in situ. Plaintiff sued under a mish-mosh of theories, claiming permanent injury from the broken screw.

After discovery, both parties moved for summary judgment.

The plaintiff relied chiefly on three depositions:

1. Plaintiff’s surgeon testified that the screw probably broke from metal fatigue, which, in turn, was caused by failure of the bone to fuse. He colorfully alluded to a “race between the bone fusing and the metal fatiguing.” In any fusion surgery, if body fusion does not occur, “ultimately at some point it is expected that the instrumentation will fail.” He thought most of plaintiff’s pain was attributable to scarring and the ordinary post-surgery healing process. Pain in the left leg would not arise from problems with the right sacral screw. It was possible that some of plaintiff’s pain came from the broken screw, but also possible that was not the case. The surgeon no longer used the SSF system (a fact exploited by plaintiff), but that was for reasons of ease of use, not because of any perceived defect.

2. Defendant’s director of product development testified that the average time for fusion could take over a year, which would involve between 1,000,000 and 2,500,000 stress cycles.
3. Plaintiff’s materials engineer testified that the screw fractured from metal fatigue that occurred after only approximately 116,000 stress cycles. (Defendant filed a Daubert challenge to this testimony, but the court didn’t need to address it.)

Defendant relied on two depositions:

1. An expert orthopedic surgeon opined that the screw broke from metal fatigue (everybody agreed on that), which was caused by failure to achieve spinal fusion, possibly a result of plaintiff’s morbid obesity. Significantly, the expert surgeon also testified that the fracture of the right screw did not cause a significant neurological defect, as plaintiff’s complaints prior to and after surgery related to her left leg (which could not be caused by the right sacral screw).

2. Defendant’s metals engineer opined that the metal fatigue was not caused by a metallurgical defect, but, rather, by “cyclic in vivo loads that exceeded those for which it was designed.” Translation: plaintiff was too big and too slow to heal. The defense expert also criticized the plaintiff engineer’s calculation of the stress cycles, because it did not account for “specific stresses.”

On this record, the court granted summary judgment to defendant. The only evidence of medical causation came from plaintiff’s surgeon, but he stated that the “significant component” of plaintiff’s pain came from “non-union of the fusion, lumbar instability, and spinal stenosis.” He didn’t think the screw was impinging on any nerves, and he expressed no definite opinion whether the fractured screw contributed to the failure of spinal fusion. In short, the court concluded that a trier of fact could find medical causation only by “speculation and conjecture” and by simultaneously ignoring the surgeon’s caveats.

This result seems perfectly correct, but it arrives almost as a pleasant surprise. There are some courts, upon being confronted by competing experts talking about spinal fusion and “metal fatigue,” that would have succumbed to “science fatigue” and would have happily passed the case along to the jury to sort out the mess. But the judge here read the record carefully and swung the gate closed on the case at exactly the right time and for the right reasons.