Anatomical Typing Of The Entire Patellofemoral Joint With Regard To Its Biomechanical Function

1. Background and PurposeCartilage lesion of the patellofemoral joint is a common and intractable disease of joint surgery, sports medicine, and ultimately will lead to patellofemoral osteoarthritis and other serious consequences. Given the characteristics of walking upright, human patellofemoral joint with the unique human anatomical features is in a complex biomechanical environment.A patellofemoral joint with a perfect biomechanical performance is like a family, and its function depends not only on a single harmonious family member, and the need for mutual understanding between the co-ordination and the adaption to the complex external environment. In the patellofemoral joint, the patella and femoral trochlear anatomy is the structure basis to its biomechanical performance, and a good congruence of the patella and femoral trochlea is an important guarantee for the patellofemoral mechanical performance. The dysplasia of the patella or the femoral trochlea can easily lead to a state of pathological patellofemoral congruency, make the patellofemoral joint unable to adapt to the surrounding complex biomechanical environment, make patellofemoral articular surface with abnormal stress distribution, which eventually led to the patellofemoral cartilage lesion and other serious consequences.However, until now the study about the patellofemoral anatomy is not only confined to the patella or femoral trochlea separately, but also not focus the morphology features which highly associate with its biomechanical performance. So the present typing of the patellofemoral anatomy can't be effectively used to judge the patellofemoral mechanical performance and to predict the incidence to of cartilage lesion. The lack of anatomical typing about the entire patellofemoral joint with regard to its biomechanical function has become the bottlenecks to an in-depth study of the patellofemoral joint. 2. MethodsIn this study, the following methods are used to initially set up an anatomical typing of the entire patellofemoral joint with regard to its biomechanical function:2.1 The initial formulation of the anatomical typing of the entire patellofemoral joint Case-control study of the patients with and without patellofemoral cartilage lesion is set to explore the patellofemoral anatomical parameters related to the patellofemoral cartilage lesion. And the results could indirectly reveal the patellofemoral anatomical parameters related to the patellofemoral mechanics. Then by observing the patellofemoral joint MR images, the initial anatomical typing of the entire patellofemoral joint is set with regard to several core parameters among the above selected anatomical parameters.2.2 Comparative study about the stress distribution among types of the patellofemoral entire anatomy by the three-dimensional finite element analysis (FEA)Three-dimensional finite element model of the type-II, III type of the patellofemoral entire anatomy were build based on in vivo magnetic resonance imaging (MRI) data. Then the stress distribution with 30°knee flexion between the two patellofemoral anatomical types was compared by finite element mathematical model, in order to verify the preliminary typing program.3. Results3.1 Case-control study shows that a total of 16 patellofemoral anatomical parameters were significantly associated with its cartilage lesion (P for trend <0.05). These parameters are as follows:①patella angle, patella lateral facet width, patella lateral facet ratio;②sulcus angle, sulcus width, sulcus depth, sulcus lateral facet width, sulcus lateral facet ratio, sulcus relative depth;③lateral patella displacement, patella epicondylar axis angle, congruence angle, patella epicondylar axis angle, lateral patellofemoral angle, patella tilt angle, width congruence, depth congruence.3.2 By observing the patellofemoral MRI images, the"sulcus lateral facet ratio"and"sulcus relative depth"are selected as the core parameters to set the patellofemoral typing. With regard to the range and tri-quantiles of"sulcus lateral facet ratio", the patellofemoral entire anatomy are divided into type I, II, III; with regard to the range and tri-quantiles of"sulcus relative depth", each of the patellofemoral entire anatomy type is divided into three sub-type and named type Ia,Ib,Ic,IIa,IIb,IIc,IIIa,IIIb,IIIc. 3.3 Patellofemoral three-dimensional solid models of its bone and cartilage about the type-II, III of the patellofemoral entire anatomy were established based on the in vivo patellofemoral sagittal MR images of two-dimensional fast low angle shot (FLASH) sequence by the manual extraction of the patellofemoral joint bone, cartilage contour and the use of AutoCAD software for precise finishing and assembly.3.4 Finite element analysis shows: with 30°knee flexion and 200 N quadriceps muscle strength, the minimum stress of type II patellofemoral surface is 0.138 MPa, the maximum stress is 2.364 MPa, the mean stress is 1.018 MPa; on the patellar side, the peak stress is 1.639 MPa, the mean stress is 1.018 MPa; on the trochlear side, the peak stress is 1.652 MPa, the mean stress is 1.021 MPa; the stress distributed homogeneously on the inner, medial and lateral side of the patellofemoral surfaces; with 30°knee flexion and 200 N quadriceps muscle strength, the minimum stress of type III patellofemoral surface is 0.209 MPa, the maximum stress is 2.509 MPa, the mean stress is 1.347 MPa; on the patellar side, the peak stress is 1.884 MPa, the mean stress is 1.270 MPa; on the trochlear side, the peak stress is 1.359 MPa, the mean stress is 1.104 MPa; the stress distributed mostly on the lateral side of the patellofemoral surfaces.4. Conclusion4.1 There are many naturally occurring variations in the patellofemoral anatomy. Through the case-control study of the patients with and without patellofemoral cartilage, several parameters correlated with patellofemoral mechanical performance are indirectly selected.4.2 The"sulcus lateral facet ratio"and"sulcus relative depth"are selected as the core parameters from the above parameters which are correlated with patellofemoral mechanical performance. Then the preliminary anatomical typing of the entire patellofemoral joint is set and this typing will be useful for effectiveness of non-invasive, accurate assessment of the patellofemoral joint.4.3 Patellofemoral three-dimensional solid models of its bone and cartilage of each patellofemoral entire anatomy type can be established based on the in vivo patellofemoral MR images, and this model can be applied to the calculation of the patellofemoral stress distribution by three-dimensional FEA.4.4 The dysplasia of the patellofemoral joint could easily lead to the state of pathological patellofemoral congruency, make patellofemoral articular surface with abnormal stress distribution, affect the articular cartilage metabolism, which eventually led to the patellofemoral joint cartilage lesion and other serious consequences.