This study evaluates high power low frequency ultrasound transmitted via a flat vibrating probe tip as an alternative technology for meniscal debridement in the knee. A limitation of this technology is thermal damage in residual meniscal tissue.

To compare tissue removal rate and thermal damage for a radiofrequency ablation device and an experimental ultrasound ablation device.

Twelve bovine meniscal specimens were treated in an identical fashion with (a) a 3.75mm 50° bipolar radiofrequency wand, Arthrocare Super Multivac 50 Arthrowand (Arthrocare Corporation, Sunnyvale, CA), operated in a free-hand manner in accordance with manufacturers instructions (n=6), and (b) an experimental flat-tipped 3mm 20kHz ultrasound probe, suspended vertically in a 500N force-controlled experimental rig (n=6).

Tissue removal rate (TRR), zone of thermal necrosis and zone of thermal alteration were calculated. Histological sections were prepared for each sample (H&E). Independent samples t-test was used to compare TRR, zone of thermal necrosis and zone of thermal alteration. Statistical analysis was performed using PASW Statistics (v.18, IBM SPSS Statistics, Chicago, IL, U.S.A.).

The mean TRR for meniscal debridement by the radiofrequency device was 5.59±1.1mg/s. This compared with a mean TRR of 4.74±1.4mg/s for debridement with the ultrasound device at settings (p=0.259, NS). Mean depth of tissue removal using the radiofrequency device was 2.21±0.26mm compared to 3.75±0.25mm (p< 0.001, ?²=0.09). Using the radiofrequency device, the mean depth of zone of thermal alteration was 1282±436µm, compared with 710±251µm for the force-controlled ultrasound device (p=0.29, ?²=0.42). For the radiofrequency device, the mean depth of zone of thermal necrosis was 64±41µm versus 97±44µm for the ultrasound device (p=0.239, NS). We observed a trend towards an increased zone of thermal necrosis and a reduced zone of thermal alteration for the ultrasound device, when compared with the radiofrequency device.

Ultrasonic debridement shows comparable thermal damage to existing radiofrequency meniscal debridement technology.

This study evaluates high power low frequency ultrasound transmitted via a flat vibrating probe tip as an alternative technology for meniscal debridement in the knee. A limitation of this technology is thermal damage in residual meniscal tissue.

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 tissue removal rate (TRR) and penetration rate (PR) was analysed. Temperature elevation in the residual meniscus was measured by embedded thermocouples and residual meniscus histological analysis. The experiment was designed using a response surface quadratic model with input variables treated as continuous, using Design-Expert v.8.0 (Stat-Ease Inc., Minneapolis, MN). Statistical analysis was conducted using PASW Statistics v.18.0 (IBM SPSS Inc., Chicago, IL).

As either force or amplitude increases, there is a linear increase in TRR (Mean±SD: 0.9±0.4 to 11.2±4.9mg/s). A corresponding increase is observed in PR (Mean±SD: 0.08±0.04 to 0.73±0.18mm/s). 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. Minimum depth of the zone of thermal alteration in residual tissue was 177.4μm.

There is an inverse relationship between both amplitude and force, and temperature elevation, with higher amplitude and force settings resulting in less thermal damage. Ultrasonic debridement shows comparable thermal damage to existing meniscal debridement technologies.

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.