The in-vitro model consisted of one fresh-frozen cadaveric radius (age: 62, male) and an interposed modular axial load transducer device (Fig. 1). The device was custom manufactured to allow for stem fixation in the diaphysis and head of the radius. An adjustable spacer was included as part of the design to allow for the effect of radial length changes on radial loads to be measured in subsequent cadaveric biomechanical studies. The device uses a commercially available axial load transducer (Subminiature Model 11, Honeywell, Morristown, NJ, USA) and the modular design allows for installation while maintaining a bone bridge to preserve the native radial head articular location.
The surgical procedure (Fig. 2) consisted of first opening a cortical window in the radial neck to expose the intermedullary canal. Bone cement was then inserted in the diaphysis and radial head, the respective stems were placed, and the load transducer was assembled in-situ, prior to cement hardening. At this time a section of the radial neck remained, termed the ‘bone-bridge,’ in order to preserve the native radial head articular location. The proximal radius osteotomy was completed after the cement hardening by removing the bone-bridge. The radius was then excised from the forearm and denuded of all soft-tissue for validation testing of the implanted device.
The radius was rigidly mounted in a clamp for digitization of the load transducer and bony landmarks using a rigid body stylus with an optical tracking system (Optotrak Certus, NDI, Waterloo, CAN). These landmarks were used to derive an anatomical coordinate system about the long-axis of the radius. The anatomical coordinate system was derived using the mid-point of the dorsal and volar aspects of the distal radial ulnar joint (DRUJ), and radial styloid point, as well as the center of the radial head. The proximal and distal coordinates of the load transducer within the bone were also digitized to determine its relative offset with the anatomical long-axis. This coordinate system allowed for alignment of the anatomical long-axis relative to the actuator of a servo-hydraulic testing frame (Instron® 8501, Norwood, MA, USA). Transducer offset was 3.61° in the anterior-posterior and 3.50° in the medial-lateral planes relative to the anatomical long-axis.
The radius was aligned and fixed in an ABS tube using the same previously digitized anatomical landmarks. Bone cement was placed in the ABS tube to secure the radius and maintain alignment of the anatomical long-axis. The ABS tube with the proximal radius protruding was mounted on a custom base within an angle jig mounted to an X-Y stage (Fig. 3). This jig allowed for a range of simulated net load vector angles of the model, and the X-Y stage ensured proper alignment of the servo-hydraulic actuator with the anatomical long-axis. The native capitellum diameter was simulated using a metal hemisphere mounted to the servo-hydraulic actuator. The base of the ABS tube pivoted on a smooth spherical bearing, aligned with the digitized anatomical long-axis of the radius, to nullify reaction moments.
Repeatability was assessed using five independent trials with the anatomical long-axis aligned with the servo-hydraulic actuator (0° position) (Fig. 4a). Loads were applied at 10 Hz from 0 to 450 N and corresponding transducer loads were collected at 10 Hz. Linear regression was used to assess linearity of the applied load and measured transducer loads for each trial. This repeatability test represented a 4.5 fold increase in desired maximum validation testing load, and was also used to assess the durability of the device under extreme loading conditions.
For validation testing of a variety of simulated net joint load directions and magnitudes, static loads were applied from 20 to 100 N at 20 N increments at angles of 10°, 20°, 30° and 40° to the anatomical long-axis. The radius model was rotated in quarter turn increments to assess loads in the anterior-posterior and medial-lateral planes, resulting in 16 separate loading conditions. Axial load transducer values were collected using custom software (Labview, National Instruments, Austin TX, USA).
External loads of 20, 40, 60, 80, and 100 N were applied using the Instron servo-hydraulic actuator. The Instron load cell provided the ‘gold standard’ load measurement, which was corrected for the angle between the actuator applied load and the anatomical long-axis, to calculate the radial axis load (Fig. 4b). The transducer axis offset of 3.61° was added to the off-axis net joint load angle in the anterior direction and subtracted in the posterior direction, and transducer axis offset of 3.50° was added in the medial direction and subtracted in the lateral direction (i.e. for 10° off-axis load in the anterior direction, \( \mathrm{radial}\ \mathrm{axis}\ \mathrm{load} = \frac{\mathrm{applied}\ \mathrm{load}\ \left(20,40,60,80,100\mathrm{N}\right)}{ \cos \left(10{}^{\circ} + 3.61{}^{\circ}\right)} \)). The measured transducer loads and known radial axis loads were plotted as Bland-Altman plots to assess variations in transducer and Instron loads, and to visualize systematic bias. Correlations between load transducer and radial axis loads at each net load position were calculated using Pearson Product Moment Correlations. Significance was set at p < 0.05.
