An analysis of the biomechanics of interference screw fixation and sheathed devices for biceps tenodesis.
BACKGROUND
This study aimed to evaluate the differences in biomechanical properties of biceps tenodesis when performed with sheathed versus unsheathed screws and also to investigate the effect of altering the pre-tension.


METHODS
Tenodesis was performed in an in vitro model using biomechanical test blocks and ovine tendons. Blocks were allocated to 1 of 5 groups which varied by method of tenodesis and cyclical loading protocol: Group A, Biosure PK screw (10-100 N), Group B: 7-8 mm Biosure Sync and Biosure PK screw (10-100 N), Group C: Biosure PK screw (10-70 N), Group D: Biosure PK (20-100 N), and Group E: custom sheath and Biosure PK screw (10-100 N). If tenodeses remained intact after 500 cycles maximum load to failure testing was performed.


RESULTS
30% of tenodeses in Group A failed prior to 500 cycles whereas none failed in the sheathed device groups (P=0.02). Using a sheathed device prevented mal-rotation. However, tenodeses in Group B were more likely to fail immediately distal to the tenodesis at a load below the anticipated maximum load to failure suggesting tendon damage during fixation. Using the custom sheath, which did not have sharp edges, resulted in a statistically significant increased maximum load to failure in Group E (348 N) when compared to Group A (228 N, mean difference 120 N, P=0.01) and Group B (253N, mean difference 95 N, P=0.0007).


CONCLUSIONS
Sheathed devices prevent mal-rotation and increase stiffness and maximum load to failure. This is further improved by reducing tendon damage at the time of tenodesis.
The Royal Orthopaedic Hospital, Bristol Road South, Birmingham, West Midlands B31 2AP, United Kingdom. Electronic address: Adnan.Saithna@nhs.net.
Adnan
A
Saithna
The University of Brunel, Department of Mechanical Engineering, Kingston Lane, Uxbridge Middlesex UB8 3PH, United Kingdom.
Mahmoud
M
Chizari
The Royal Orthopaedic Hospital, Bristol Road South, Birmingham, West Midlands B31 2AP, United Kingdom.
Guy
G
Morris
The Royal Orthopaedic Hospital, Bristol Road South, Birmingham, West Midlands B31 2AP, United Kingdom.
Cameron
C
Anley
The University of Brunel, Department of Mechanical Engineering, Kingston Lane, Uxbridge Middlesex UB8 3PH, United Kingdom.
Bin
B
Wang
The Royal Orthopaedic Hospital, Bristol Road South, Birmingham, West Midlands B31 2AP, United Kingdom.
Martyn
M
Snow
D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws; D004867:Equipment Design; D066298:In Vitro Techniques; D012756:Sheep; D013710:Tendons / Q000502:physiology* / Q000601:surgery*; D053656:Tenodesis / Q000295:instrumentation* / Q000379:methods; D013718:Tensile Strength; D016474:Weight-Bearing
Biceps tenodesis; Biomechanics; Interference screw; Sheathed device