Does Cyclical Loading Affect Antibiotic Elution and Strength of Articulating

Cement Knee Spacers?

BA Rogers1, F Middleton2, N Shearwood-Porter3,

S Kinch3, A Roques4, N Bradley5, A Taylor4, M Browne3

1. Mount Sinai Hospital, Toronto

2. Kingston Hospital, UK

3. School of Engineering Sciences, University of Southampton, UK

4. Aurora Orthopaedics, UK5. Royal Surrey County Hospital,

UK.

Study Rationale

To date, little known on how cyclical fatigue

loading affects antibiotic elution from cement

spacers

Aim

To compare the differential effect of antibiotics

on in-vitro elution with static or dynamic loading

Null hypothesis

Cyclical loading of cement spacers has no

significant effect on antibiotic elution

Sample Preparation• Hand mixed Palacos R bone cement

• Biomet Stage One 70mm silicon spacer moulds

Methodology: Specimen Preparation

• 4 spacers for each concentration of antibiotics were made for dynamic loading

• Two spacers for each concentration were made as controls

• Immersed in Phosphate buffered solution (PBS)

• 5ml samples of PBS taken over a set 48 hr schedule

Concentration Mass of Vancomycin per

40g cement

Mass of Tobramycin per 40g cement

Low 1g 1.2g

Medium 1g 2.4g

High 1g 3.6g

Methodology: Dynamic Loading

Methodology: Sampling Rate

• Knee spacers immersed in solution; 3 x 5ml samples taken as per schedule

• Measurement of antibiotic elution by ELISA

Time Elapsed No. of cycles

Test Control

5 minutes 0 0

2 hours 0 0

4 hours 7000 0

6 hours 14000 0

22 hours 14000 0

24 hours 21000 0

26 hours 28000 0

28 hours 35000 0

Vancomycin Elution Control Group

• Rapid initial release• Levelling of concentration after ~ 400mins• Vancomycin elution increases with increasing dose of

tobramycin

Tobramycin Elution Control Group

• Similar profile• Tobramycin elution increases with increasing dose of tobramycin

Dynamic Results:

Low Concentration

1.0g Vancomycin,

1.0g Tobramycin

Vancomycin Release Tobramycin Release

Dynamic Results: Medium Concentration

1.0g Vancomycin,

2.4g Tobramycin

Vancomycin Release Tobramycin Release

Dynamic Results: High Concentration

1.0g Vancomycin,

3.6g Tobramycin

Vancomycin Release Tobramycin Release

DiscussionThis study confirms…..

Static & Dynamic Spacers

• Standard elution pattern :

– rapid initial antibiotic release

– steady state release at 400mins

• Increasing tobramycin in cement

– enhances vancomycin elution

– enhances tobramycin elution

DiscussionThis study adds…

Cyclical loading increases antibiotic elution

• P<0.05 (ANOVA)

• Tobramycin potentiates effect of cyclical loading

• Dose-related manner

Discussion•Masri et al1, found significant increase in static elution of vancomycin when dose of tobramycin was increased

•Penner et al2, propose ‘passive opportunism’; whereby antibiotics elute by leaking out from voids and cracks.

•Tobramycin - increases void formation, and thus increasing elution

•Need further results to investigate this1. Masri et al, 1998, J. Arthrop., 13(3), 331-338

2. Penner et al, 1996, J. Arthrop., 11(8), 939-944

Null hypothesis

In vitro cyclical loading of cement spacers has no significant effect on

antibiotic elution

Refute for Tobramycin and Vancomycin,

with statistical significance (p<0.05,

ANOVA)

ConclusionDynamic Cement Spacers

• Mechanical and soft tissue benefits well known

• This study provides evidence for the biological benefit

of articulating cement spacers in revision surgery

• Future work

– zirconium dioxide replaced by CaCO3

– Different antibiotic combinations or anti-fungals?

– Load to failure & cement porosity

Thank YouAcknowledgements