Abstract
Background: The mechanism of tissue removal and residual tissue damage for ultrasonic ablation instruments have not been adequately investigated. In particular, the relationship between applied force and amplitude of distal tip displacement as determinants of cutting effect and residual tissue damage has not been clearly defined. Recent clinical studies have highlighted the potentially deleterious thermal and mechanical effect of ultrasonic energy in residual tissue.
Aims: To evaluate the role of ultrasonic tissue resection as an alternative to mechanical shaver and electrosurgical resection for orthopaedic applications. We aim to investigate factors influencing material removal rate (MRR), cutting rate (CR) and thermal damage for meniscus tissue resection using an experimental 20kHz ultrasonic ablation device.
Methods: An experimental force controlled testing rig was constructed using a 20kHz ultrasonic probe suspended vertically from a load cell. Ex-vivo bovine meniscus samples were harvested from knee joints and cut into uniform 16mm discs. Effect of variation in force (2.5–4.5N) and amplitude of distal tip displacement (242–494μm peak-peak) settings on material removal rate (MRR) and cutting (CR) was analyzed. Time-discrete temperature elevation in the meniscus was measured by embedded thermocouples and infrared thermography. Statistical analysis was conducted using SPSS v.11.0 (SPSS Inc., Chicago, IL). The experiment was designed using a response surface quadratic model with both input variables treated as continuous, using Design-Expert v.7.1.3 (Stat-Ease Inc., Minneapolis, MN).
Results: As either force or amplitude increases, there is a linear increase in MRR (Mean±SD: 0.9±0.4 to 11.2±4.9mg/s). A corresponding increase is observed in CR for increases in force and amplitude (Mean±SD: 0.08±0.04 to 0.73±0.18mm/s). Conversely, there is an inverse relationship between both force and amplitude, and temperature elevation, with higher force and amplitude settings resulting in less thermal damage. Maximum mean temperatures of 84.6±12.1°C and 52.3±10.9°C were recorded in residual tissue at 2mm and 4mm from the ultrasound probe-tissue interface respectively.
Conclusions: Although high power low frequency ultrasound is capable of meniscal resection, key limitations of this technology are low MRR rate and thermal damage. The mechanism of removal is primarily thermal, with tissue temperatures reaching potentially dangerous levels. Control of user force and amplitude of tip displacement settings in ultrasonic instrument design can maintain temperature peaks below critical temperatures of thermal necrosis during operation.
Correspondence should be addressed to Mr Richard Wallace at Musgrave Park Hospital, 20 Stockman’s Lane, Belfast BT9 7JB, Northern Ireland.
