OTA 1997 Posters - Shoulder Fractures

Biomechanical Fixation Strength in Surgical Neck Fractures of the Proximal Humerus

David J. Clare, MD, Christopher K. Hersh, MD, Kyriacos Athenasiou, PhD, PE, Michael A. Wirth, MD

UTHSCSA, San Antonio, Texas, USA

Purpose: The purpose of this study was to biomechanically assess and compare the fixation strengths of three accepted and utilized fixation techniques for surgical neck fractures of the proximal humerus.

Methods: Eighteen (18) fresh frozen cadaveric humeral specimens donated from the Musculoskeletal Transplant Foundation (MTF: Holmetil, New Jersey) were collected and dissected free of any soft tissue. The donors ranged in age from 18 - 64 years with a mean age of 43.5 years. Their medical histories were free of any malignancy, autoimmune or metabolic bone disease. A reproducible, oblique osteotomy was created at the surgical neck in each specimen and they were randomly assigned to one of three testing groups: 1) the AO T-plate, 2) multiple AO pins, and 3) combination screw-tension band technique.

The group repaired with multiple AO pins were done so by a method described by Jakob. Four terminally threaded 2.5 mm pins were utilized with two pins driven from the lateral cortex superiorly across the fracture and into the humeral head. A single pin was guided from the anterior cortex across the fracture into the head, and a final pin directed inferiorly from the greater tuberosity across the fracture to penetrate the medial cortex. The combination screw-tension band group was repaired by a method described by Cornell et. al. Two 18-gauge cerclage wires were fashioned in a figure-of-eight configuration with supplemental lag-screw fixation with a 6.5 mm cancellous screw.

Following fixation, each specimen was mounted to the Materials Testing System (858 Servo-Hydraulic Bionix Testing System; MTS, Systems Corp., Minneapolis, MN) at approximately 30 degrees from the vertical axis of the bone. A 1/4" drill hole was placed through the greater tuberosity allowing passage of a stainless steel rod which was connected to cables suspended by a load cell. Testing was then performed with a tensile force and rate of displacement of 1 mm/s until failure occurred. For this study, failure was defined as displacement of 1.0 cm. A force-displaced curve with 10 data points/s was generated for each of the specimens.

Results: The AO T-plate provided the strongest fixation of the methods tested with a mean force at failure of 1079.7 N. The combination screw-tension band technique failed at a mean force of 636.5 N, followed by the multiple AO pin group with a mean force of 203.7 N at failure. Statistical significance was examined using analysis of variance and multiple comparison tests of the means (p<0.05).

Discussion and Conclusion: Relatively few studies have assessed biomechanical fixation strengths associated with surgical neck fractures of the proximal humerus. Koval et. al. performed a series examining the strengths of multiple fixation techniques for both osteopenic and non-osteopenic specimens. The T-plate provided the strongest fixation in the non-osteopenic specimens while multiple Schanz pins provided the strongest fixation in the osteopenic group. Sehr and Szabo compared fixation strengths between the AO T-plate and a fashioned semi-tubular blade plate and they were not able to demonstrate statistical significance.

This study compared the tensile strengths of three accepted and utilized fixation techniques for the treatment of displaced surgical neck fractures of the proximal humerus. Not surprisingly, the AO T-plate provided the strongest fixation of the methods tested. However, the screw-tension band group demonstrated excellent tensile strength and has the added advantage of less soft-tissue dissection and periosteal stripping. The importance of this pertains to clinical situations where one method of fixation may be favorable over another. In an osteoporotic individual with poor bone stock and a borderline medical status, avoidance of excessive soft-tissue dissection and periosteal stripping as would be required for application of the AO T-plate has obvious advantages.