Mechanical characterization of the ligaments in subject-specific models of the patellofemoral joint using in vivo laxity tests

BACKGROUND

The purpose of this study was to propose a methodology for mechanical characterization of the ligaments in subject-specific models of the patellofemoral joint (PFJ) of living individuals.

METHOD

PFJ laxity tests were performed on a healthy volunteer using a specially designed loading apparatus under biplane fluoroscopy. A three-dimensional (3D) parametric model of the PFJ was developed in the framework of the rigid body spring model using the geometrical data acquired from the subject's computed tomography and magnetic resonance images. The stiffness and pre-strains of the medial and lateral PFJ ligaments were characterized using a two-step optimization procedure which minimized the deviation between the model predictions and the calibration test results. Sensitivity analyses were performed to investigate the effect of mechanical properties of the non-characterized model components on the characterization procedure and its results.

RESULTS

The overall findings indicate that the proposed methodology is applicable and can improve the model predictions effectively. For the subject under study, ligament characterization reduced the root mean square of the deviations between the patellar shift and tilt predicted by the model and obtained experimentally for the validation laxity test (from 6.2 mm to 0.5 mm, and from 8.4° to 1.5°, respectively) and passive knee flexion test (from 1.4 mm to 0.3 mm, and from 2.3° to 1.3°, respectively). The non-characterized mechanical properties were found to have a minimal effect on the characterization procedure and its results.

CONCLUSION

The proposed methodology can help in developing truly patient-specific models of the PFJ, to be used for personalized preplanning of the clinical interventions.

PATELLOFEMORAL ARTHROPLASTY CHANGES THE TROCHLEAR GROOVE ANGLE

High early failure rates occur in the treatment of isolated symptomatic patellofemoral arthritis with commercially available patellofemoral arthroplasty (PFA) prostheses. We postulate that PFA changes the trochlear groove angle, thereby causing patellar maltracking, catching and pain. We examined the extent of this change in trochlear groove angle by virtually implanting five commercially available patellofemoral prostheses into two 3D reconstructed knees, one with a normal and the other with a dysplastic trochlea. The axial and coronal trochlear groove angles were measured pre- and post PFA for the five different prostheses in both the normal and the dysplastic knee. Post PFA, the trochlear groove angle changed from the original in both the axial and coronal planes for all the prostheses in both the normal and the dysplastic knee. The trochlear groove change is dependent on the design of the specific prosthesis. To avoid excessive changes post PFA, both the wide variation of changes between different generic PFA prostheses, as well as the wide variation in patient femoral anatomy should be considered.

The effect of axial rotation of the anterior resection plane in patellofemoral arthroplasty

BACKGROUND

Patellofemoral arthroplasty (PFA) has a small but definite place in replacement surgery of the knee, especially in young patients. The main surgical considerations in PFA are the patient's anatomy, the type of prosthesis and the surgical technique. The surgical technique and PFA success rely heavily on the anterior resection. In this study we investigate the effect of axial rotation of the anterior resection plane.

METHODS

We tested the outcome of PFA fit based on resection footprint measurements, axial and coronal groove angles, and lateral trochlear inclination (LTI) angle in a virtual PFA model. The range of anterior resection plane axial rotations was from five degree internal to five degree external with an increment of one degree.

RESULTS

Axial rotation of anterior resection plane changes the resection footprint dimension, which leads to coronal rotation of the femoral component. External rotation of the resection plane results in valgus rotation of the trochlear groove and decreased LTI after PFA and the opposite was observed for internal rotation.

CONCLUSION

Our study showed that by changing the axial rotation of the anterior cut, the coronal groove of the prosthesis can be altered to lie more closely with the native groove line without compromising the prosthesis-cartilage transition.