Refixation stability in shoulder hemiarthroplasty in case of four-part proximal humeral fracture

Abstract

Primary stability of refixated fractures in case of shoulder hemiarthroplasty is a prerequisite to restore physiological glenohumeral joint function. Clinical observations often show a secondary dislocation and subsequent resorption of the bony anchor points like the greater and lesser tuberosity at the rotator cuff tendons. This failed integration leads to impaired glenohumeral load transmission and subsequent reduction of mobility. As a consequence, the optimisation of refixation methods is crucial for a better clinical outcome. To prove the stability of refixation techniques, a Finite Element fracture model was built. Resulting stresses at the bone surface and fragment migration relative to the prosthesis shaft were studied. The results of the calculations show that the isolated tuberosities show unstressed bone regions compared to the intact model. This circumstance may explain the clinically detected bone resorption due to the absence of mechanical stimuli. Furthermore, a cable guidance through lateral holes in the middle part of the proximal prosthesis results in a lower fragment displacement than a circumferential fixation method surrounding the entire proximal bone.

Review of fixation techniques for the four-part fractured proximal humerus in hemiarthroplasty

“…Connecting two circumferential oriented cables (one placed around the fragments, one around the shaft) by another cable presumably leads to enhanced stiffness [48] (Figure 2). …”

“…In other biomechanical tests, a torque was introduced at the humeral bone which induced a rotation around the humeral longitudinal axis to apply passive muscular tension [29,30]. A further investigation used a numerical approach to mathematically determine the strength of the fixation by means of a Finite Element Analysis [31]. These biomechanical investigations show that substantial efforts have been made to find an appropriate and stable fixation technique for a four-part fracture.…”

The Applications of Finite Element Analysis in Proximal Humeral Fractures

“…What they discovered helped explain why certain types of osteosynthesis fail were due to tuberosity reconstruction failures. If the tuberosity was placed in an unstressed isolated circumstance, the bone would be absorbed due to the absence of mechanical stimuli, described by Baumgartner after performing a FEA test [ 88 ]. What Baumgartner observed corresponded to the theory described by Wolff that if the loading on a bone decreases, the bone will become less dense and weaker due to the lack of the stimulus required for continued remodeling.…”

Shoulder biomechanics: today’s consensus and tomorrow’s perspectives

“…Suarez et al [17] and Baumgartner et al [2] used finite element models to analyze the relation between endoprostheses and the surrounding bone. Suarez studied how a degenerated cuff can affect the initial stability of a cementless glenoid implant, while Baumgartner evaluated the quality of a refixation technique in the case of shoulder hemiarthroplasty after proximal humeral head fracture.…”