Forensic Engineering Analysis and Daubert Challenges to Prosthetic Fastener Failure
by Roger L. Owens, P.E. (NAFE 412F)


It has become relatively commonplace in those cases where trial appears to be eminent that a Daubert Challenge of an expert occurs. Once one expert is challenged, it is not unusual that all experts on both sides of the docket are challenged. In the past, case value appeared to be the motivating issue when considering a challenge of an expertís qualifications and/or methodology. The challenge is now being utilized more frequently in all courts and at lower case value levels. If an expert testifies frequently the probability of the expert being restricted and/or struck increases. Once challenged, the expert is dependent on his client to advise him of the challenge as well as the grounds for the challenge, to assist in the preparation of an affidavit and to prepare for testimony before the court. The expert, of course, is totally dependant upon the legal competence of his client and the fairness of the court. This article is intended to document the forensic analyses of a mechanical failure of a bolted joint in a foot prostheses. Although the failure was relatively fundamental from an engineering perspective, multiple Daubert Challenges of both qualifications and methodology complicated the case.

Case Overview
From a pure failure scenario this mechanical failure was relatively easy to analyze. A mechanical joint consisted of two independent surfaces clamped together by a Ĺ inch counter sunk 304 stainless steel bolt with a #8 hex drive torqued to 45 ft. lbs. The joint, clamping forces and the bolt were designed and specified by the manufacturer to prevent rotation between the two surfaces. The joint did fail catastrophically under normal use and allowed uncontrolled rotation between the two surfaces. The mechanical coupling at this joint held together the upper and lower halves of a foot prosthesis hence when the rotation of the coupling occurred the foot rotated causing the individual to fall. The individual injured his remaining foot and it ultimately had to be amputated. The replacement prosthesis which was supplied for the failed unit contained a different and unique design that appeared to be far superior to the original design that had failed. Bench testing was made and did validate the hypothesis that the replacement design was superior to the original design from an anti-rotational perspective. It should be noted that although the original motivation for the test project was to prove or disprove the hypothesis that the replacement design was a superior design, the testing showed unequivocally that the original design was inappropriate and unsafe for its intended use, hence defective.

Case Chronology
The failed joint as well as the joint with a new design was provided for examination. The joints had previously been disassembled by the manufacturers representatives and a video tape complete with audio was provided to document their disassembly. The joint coupled the upper and lower sections of the prostheses together such as to inhibit rotation. The surfaces on the failed joint (see figure 1) showed severe galling consistent with rotation under load. The damage to the surfaces was consistent with the failure description of the plaintiff.

Figure 1

The new design shown in figure 2 had a knife blade on one surface and a composite on the opposite surface. Rotational forces tend to push the polymer forward of the blade such as to increase the clamping forces of the joint. Hence the new design from an anti-rotational perspective is superior. Figure 3 shows a comparison of the two designs.

A short report was prepared after the initial evidence examination to document the rotational failure that had occurred to the original design. Further, the report stated that the new design is superior from an anti-rotational perspective. After submitting the preliminary report, the first deposition was scheduled.

Figure 2

Figure 3

The majority of the questions in the first deposition had to do with the authorís website, degrees, courses and electrical experience, Any and all mechanical experience, education, and qualification was just glossed over. The thrust of the deposition was to document the electrical qualifications and experience of the author and the lack of mechanical engineering references in the website. Of course, the tactic was to prepare for Daubert challenges to qualifications.

Subsequent to the deposition the first Daubert challenge was petitioned to the court and a hearing occurred. The direct testimony highlighted mechanical experience, education and qualifications. The cross-examination was a repeat of the first deposition strategy; website, degrees, forensic experience. As a result of the hearing the court asked for testing to occur in order to validate or invalidate our opinion that the new design was superior to the original design. A protocol was developed and the hypothesis was tested in the presence of all parties.

Test Results and Conclusion
Two sets of joints of the original and new design were tested. A jig was manufactured out of steel so that the joints could be restrained (see figure 4), assembled and tightened to factory specs and then tested by forcing rotation. (See figure 5) A second test identical to the first test was conducted to assure repeatability and to validate the test results. The factory specifications were to torque the restraining bolt to 45 ft. lbs. Under that loading condition the surfaces should not rotate relative to each other. The test demonstrated that the original design consistently failed at a load equivalent to 60 ft. lbs of torque. The new design was tested at the initial load of 45 ft. lbs and then at 35 ft. lbs. and 25 ft. lbs and did not fail at 100 ft lbs. of torque. Clearly the new design was superior as to rotational resistance.

Figure 4

Figure 5

A detailed test report was issued and a second deposition then occurred. Again, the website and degrees were emphasized as well as an absence of publications on bolt failure. Defendants disagreed with the protocol, claimed that their engineers did not actually participate in the tests and the tests were invalid since the twin beam torque wrench that was used had not been calibrated. After the testing, five additional days were spent in the laboratory examining the four component assemblies that were tested. Digital and microscopic photos as well as Scanning Electron Microscopic analysis occurred with thousands of photographs.

The Second Challenge
The second challenge then occurred attacking the same qualification issues as well as the methodology of the test project. A lengthy affidavit was prepared to document:

  • Mechanical Education in detail
  • Designation as ME by NSPE and licensing in Canada as a Mechanical Designer
  • Chief Design Engineer of Mechanical Contracting Company
  • Project Manager for Bolting division of Furmanite America
  • Critique of the video of the Manufacturers representativeís disassembly of failed components
Court Testimony
During trial testimony the same line of questioning surfaced on cross-examination regarding the website, degrees, and experience in mechanical forensics etc. The extensive preparation necessary for the affidavit provided the necessary ground work for acceptance by the court in all respects. Following our testimony a generous offer for settlement was tendered and refused by the plaintiff. The jury ultimately found for the defense.