Introduction

Surgical drill-bits are used in a raft of procedures, from trauma, joint reconstruction to Arthroplasty. Drilling of bone is associated with the conversion of mechanical work energy into shear failure of bone and heat generation, causing a transient rise in temperature of hard and soft tissues. Thermal insults above 47°C sustained for one minute or more may cause osteonecrosis, reduced osteogenic potential, compromise fixation and influence tolerances with cutting blocks. Drill design parameters and operational variables have marked effects on cutting performance and heat generation during drilling. Dulling and wear of the cutting surfaces sustained through repeated usage can significantly reduce drill bit performance. Deterioration of cutting performance substantially increases the axial thrust force required to propel the cutting face through bone, compromising surgeon control during drilling and increasing the likelihood of uncontrolled plunging, cortical breakthrough and improper placement of holes as well as other jigs.

Introduction: The majority of midshaft humeral fractures will achieve a satisfactory outcome with non-operative management. However, internal fixation is occasionally required to assist with rehabilitation, particularly in multiply-injured patients. Although the clinical risks and benefits of the locking plate and humeral nail are well known, there is a paucity of data comparing their mechanical properties.

The aim of this study was to determine the torsional and 4-point bending properties of a midshaft humeral osteotomy reconstructed with either an intramedullary nail or locking plate.

Methods: 19 fresh cadaveric humeri were DEXA scanned to ensure similar BMD. Non-destructive 4-point bending was performed on the intact bone to determine stiffness in the sagittal and coronal planes. Load was applied using an MTS MiniBionix 858 (Mechanical Testing Systems, MN) at a rate of 1 mm/min to a maximum of 450 N.

A transverse midshaft osteotomy was created and a spacer ensured a constant 3-mm gap between the bone ends. Reconstruction was performed with either

Trigen humeral nail (Smith & Nephew, TN) – 10 specimens

Non-destructive 4-point bending was repeated, and then each humerus was embedded in a low-melting point alloy proximally and distally for torsional testing. Torque was applied at 5 deg/min until failure. Maximum torque, maximum angle and stiffness were calculated.

All data were analysed with SPSS for Windows (SPSS Inc., Il) using ANOVA.

Results: One specimen in the locking plate group fractured during plate application and was excluded from the study. Non-destructive bending tests showed no significant difference in stiffness of the intact bones between the two groups.

4-point bending: the bones reconstructed with the intramedullary nail were ~50% as stiff as the intact state in both planes. There was no statistically significant difference in stiffness between the intact bones and those reconstructed with the locking plate.

Torsional testing: the locking plate specimens were 3 times as stiff as the intramedullary nail specimens (P< 0.05) and failed at twice the torque (P< 0.05).

Discussion: Humeral intramedullary nails are reported to have an advantage over plates under axial loading (Chen et al, 2002). However, this study demonstrates that locking plates are superior to intramedullary nails in torsion and four-point bending. Although the clinical situation often dictates the most appropriate management, locking plates should be considered in patients when torsional or four-point bending loads are expected to predominate in the post-operative period.

Introduction: A cosmetic deformity does not always occur after a biceps tenotomy. The anatomical restraints preventing distal excursion of the long head of biceps tendon following tenotomy have not previously been described. This study aims to evaluate the biceps sheath and its potential role as a restraint to distal excursion of the biceps following tenotomy.

Methods: Fifteen fresh cadaveric specimens were dissected free of overlying soft tissues to reveal the rotator cuff, biceps sheath and long head of biceps muscle belly and tendon. Eight specimens were used for gross anatomical analysis. Measurements of the length of the biceps sheath on the humeral (bone) side and tendon side were made using a digital caliper (Mitutoyo, Japan). The long head of biceps tendon was then released from the glenoid labrum and the excursion of the stump relative to the rim of the articular surface measured. The biceps sheaths of two specimens were used for histological analysis.

Seven specimens were used for mechanical analysis. A humeral osteotomy was performed distal to the insertion of pectoralis major, leaving intact the biceps sheath and the muscle belly of long head of biceps. The proximal humerus was attached to a custom-designed jig and the muscle belly of biceps grasped in cryogenic grips. Specimens were loaded on an MTS 858 Bionix mechanical testing machine (MTS Systems, MN) in uniaxial tension at a rate of 1 mm/sec until failure was observed.

Results: The biceps sheath surrounds the long head of biceps tendon and inserts into the bone of the proximal humerus. It is trapezoidal in cross-section, with a mean length of 75.1 mm on the bone side and 49.3 mm on the tendon side. The average excursion of the stump was to within 2.8 mm of the rim of the articular surface.

Histological examination of the biceps sheath revealed membranous tissue consisting of loose soft tissue with fat and blood vessels. Synovial tissue was also identified. The sheath was seen to loosely attach to the biceps tendon, with a more intimate attachment to the periosteum.

The mean force to pull the long head of biceps tendon out of the sheath 102.7 N (range 17.4 N–227.6 N)

Discussion: The biceps sheath is a consistent structure intimately associated with the biceps tendon. It appears to contain blood vessels which provide nutrition to the tendon, similar to the vincula of flexor digitorum pro-fundus. Mechanical testing reveals that a substantial force is sometimes required to pull the biceps tendon from the sheath. This may explain why biceps tenotomy does not routinely result in a “Popeye” biceps.