Minimally invasive lateral plate placement for metadiaphyseal fractures of the humerus and its implications for the distal deltoid insertion- it is not only about the radial nerve. A cadaveric study.

INTRODUCTION: Minimally invasive lateral placement of plates on the humerus may be associated with a risk of injury to the radial nerve. Whereas this potential complication has been investigated in several studies, there is no data regarding potential injuries to the distal insertion of the deltoid muscle when the plates are passed distally in a submuscular tunnel.

METHODS: Minimally invasive plate placement was performed on eight arms in fresh cadavers. A lateral deltoid split approach was made and the plates were introduced in an antegrade submuscular manner from proximal to distal. A lateral distal incison was made to adjust the position of the plate at the lateral aspect of the humerus without formal exploration of the radial nerve. The arms were dissected to identify the seven intramuscular tendons of the deltoid and their insertions at the humerus. The position of the plate and its relation to the intramuscular tendons of the deltoid was explored. Furthermore, potential injuries to the axillary and radial nerve were investigated. Damage to the brachialis muscle its interference with plate positioning were explored.

RESULTS: The distal deltoid insertion was affected in all eight examined arms. The two most anterior and two most posterior segments were intact in all. In two arms, the third intramuscular tendon was perforated. In three specimens, the insertion of the fourth segment was damaged. The fifth segment was partially disrupted in three arms. Overall, injuries to the intramuscular tendons were limited to one tendon in all arms. Partial brachial muscle entrapment underneath the plate was observed in four specimens. The axillary nerve was not damaged in any of the examined arms. The radial nerve was entrapped between plate and humeral shaft in one case.

CONCLUSIONS: Lateral plate placement in MIPO technique damages central parts of the distal deltoid muscle insertion. However, the most anterior and posterior tendons are not involved and the clinical significance on muscle function remains unclear. Introduction of the plate without prior distal incision and elevation of the brachial muscle may be associated with partial entrapment of the brachial muscle and a higher risk of injuring the radial nerve.

LEVEL OF EVIDENCE: Experimental study.