Guang-Quan Zhou,Hong-Lai Zhang

1 The First Affiliated Hospital,Guangzhou University of Chinese Medicine,Guangzhou,China.2 School of Medical Information Engineering,Guangzhou University of Chinese Medicine,Guangzhou,China.

Background

Femoral head necrosis(FHN)is a pathological process,with the characteristics of initially injury of necrotic bone trabecular structure that occurs in the load-bearing zone of the femoral head and the subsequent repair process of the injured bone tissue.As the damage-repair process continues,it results in structural changes of the femoral head,collapse and deformation,and finally osteoarthritis(OA)and joint dysfunction.In the theroy of traditional Chinese medicine(TCM),FHN is called Gushi,which was first recorded in Huangdineijing published on Xihan Dynasty of China(the third century B.C.).

FHN is a debilitating disease which generally affects young adults,resulting in femoral head collapse and OA.In Asia,there is approximately 40%of the primary hip replacement caused by OA after FHN[1].In addition to drug therapy,the conventional conservative management methodshaveinvolved restricted weight-bearing,bed rest,surgery and traction therapy.However,weight-bearing and bed rest may result in osteoporosis,slow metabolism,muscle atrophy and poor clinical curative effect[2,3].Several hip preserving surgery methods[4-9]have been developed to interrupt the disease pathway of collapse and OA after FHN,averting the need for hip replacement.Isolated core decompression will accelerate a collapse and OA progression of the femoral head because of the lack of repaired materials and biomechanical structural support during the healing of the necrosis region[10,11].Free vascularized fibular grafting can provide immediate structural support and vascularity,but it is often associated with serious trauma,technical difficulties and longer recovery time. Transtrochanetric rotational osteotomy[7]can improve the biomechanical properties in daily activity and effectively decreased the average stress by either anterior or posterior rotation.However,it is a technically demanding procedure associated with high complication risks.

Clinical doctors and researchers share a common goal of choosing safe and effective treatment procedure for protecting the femoral head of the patients with FHN[12,13].Non-drug therapy is the commonly therapeutic method in the field of TCM rehabilitation therapy,including rest,reasonable exercise and joint function exercise.Lower extremity abduction(LEA)represents an effective hip preservation method in TCM rehabilitation therapy,which can relieve pain,eliminate inflammation and preserve the shape of the femoral head without surgery intervention,restricted weight-bearing and bed rest.Enhanced LEA motion is associated with a decrease in postnecrotic OA.This therapy works help to move the necrotic portion away from the weight-bearing area to reduce stress concentration,improve the biomechanical properties via full weight-bearing in daily activity[14],enhance the ability of bony healing via increasing stress stimulation[15,16],and decrease the peak contact hip stress[17].However,these conclusions are mainly based on observational clinical experience,which makes it controversial.

Clinical practice concept requires proof in theory.We postulated that the enhanced LEA motion during gait may reduce the risk of collapse of FHN via reconstructing the stress transfer patterns and protect against OA progression.To test this hypothesis,thirty parametric computational models were generated to investigate biomechanical behavior changes of FHN by increasing the lower extremity abducent angle,offering scientific basis on choosing the proper treatment for the exact classification.

Material and methods

Japanese Investigation Committee(JIC)classification

In 2001,the JIC revised diagnostic criteria to clarify the definition of osteonecrosis of the femoral head[18,19].According to the JIC classification criteria,FHN can be classified into subtypes A,B,C1 and C2,based on the location of the lesion in the weight-bearing area(as shown in Figure 1).Type A lesions occupy the medial one-third or less of the weight-bearing portion,while type B lesions occupy the medial two-thirds or less of the weight-bearing portion.Type C1 lesions occupy more than the medial two thirds of the weight-bearing portion without extending laterally to the acetabular edge.Type C2 lesions occupy more than the medial two-thirds of the weight-bearing portion and extend laterally to the acebtabular edge.

Figure 1 JIC classification of FHN[18]

Parametric modeling

Five computational models were generated as a healthy model and four subtype models of FHN based on JIC classification criterion.The parametric definition of the models enabled the study of the effect of abduction on stress transfer.Single-legged stance was considered as a representative body position for the primary models.We used an abducent angle β (β =0°,10°,15°,20°,25°and 30°)as a variable[20,21]along the anteroposterior axis of femoral head to investigate stress transfer path(STP)and contact stress.The abducent variants were depicted schematically in Figure 2.Finally,a total of thirty different computational models were used to simulate six hip abductions with fivedifferent progresses of FHN.

Boundary conditions

A ground reaction force equivalent to body weight was exerted on a rigid plate that tied to the distal femur.Constrains were applied to pubic symphysis and sacroiliac joint.All six degrees of freedom were constrained to zero for all cases described above.Seven muscles were modeled as axial connectors and muscle forces were depicted in literature[22].In this study,the muscleforceswere postulated to be constantsduring LEA gait.The parametric finite element analysis was performed using ABAQUSV6.13.All models were input to generate isotropic 10-node tetrahedral elements.The mesh sizewas3 mm.

Figure2 Lower extremity abducent angle

Figure 3 Photograph,radiograph and computational results of human proximal femur

Model validation

The model validation of this study was performed by comparing the median coronal femoral head nephogram and the median coronal femoral trabecular bone profile of normal subjects.

Results

Model validation

The major function of principal compressivetrabeculae in the proximal femur is loading principal stress.The stress trabecula carries the conduction of principal stress,and the direction of conduction is from femur head bearing area to femoral moment(Figure 3C),which correlates well with the trabecular features in the cross-sections of the cadaver bone(Figure 3A)and the bone density distribution(Figure 3B)[27-29].These phenomenons conform to Wolff’s law:the structure and function of bone are interdependent[30].Hence,the biomechanical behavior of this model was considered valid.

Stress transfer path

In this study,STPis the measurable change of load borne in femoral head during LEA motion.The effects of stress transfer characteristics were evaluated and the coronal contours of proximal femur on stress were plotted.As shown in the Figure 4,on normal conditions the STPs moved from weight-bearing region to lateral column with increasing LEA angles;on stance conditions(β=0°)the STP of type A(Figure 4B)showed strong similarities with the healthy level(Figure 4A);STP distribution was from the top of the femoral head to the calcar;the damage area of STP of type B(Figure 4C)was approximately 25%of the healthy condition.While STPs in both type C1 and C2(Figure 4D and Figure 4E)were broken off.The damage areas of STPof type C were more than 50%of the healthy condition.As the increase of the abduction angle,STPs of the femoral head were increasing restoration;type B could be transformed into type A,while type C1 could be transformed into type B;type C2 could be transformed into type C1.

Contact stress

Contact stress is an important biomechanical index in evaluation of OA progression between pre-treatment and post-treatment[23,24].Since elevated contact stress may accelerate the progression of OA,the peak values and average values of the contact stress were used for the analysis.The contact pattern varied according to the simulated loading activity.Figure 5 show the relationship between the LEA motion and the likelihood of OA progression.Greater LEA motion would increase the peak contact stress underlying healthy condition,which is in accordance with previous literature[25,26].However,in this study,there was no significant difference in peak contact stress between the four settings of FHN and normal femoral head.The average values of the contact stress in four setting lay within the range of values corresponding to normal setting.

Figure4 Stresstransfer path

Figure5 Thecontact stressof articular cartilage

Discussion

In recent years,various hip preserve methods have been developed to maintain the femoral head of the patients and reconstruct the biomechanical stability to prevent the FHN deteriorated into OA in the pre-collapse stage.In some way,these procedures delay or interrupt the likelihood of hip replacement.Hence,femoral head preservation methods still play an important role in the management of FHN.Enhanced LEA motion is an effective femoral head preservation method,which could stop the progression of FHN without reducing weight-bearing and bed rest and surgery intervention.Enhanced LEA motion improves the biomechanical properties of bone density and muscle force as well as speeds up the metabolism during cyclic loadings in daily activity.However,the best classification for LEA intervention and itsmechanism are still unclear.

Computational biomechanical approach has the potential to quantify the relationship between hip morphology,cartilage mechanics and OA[31,32].Computational techniques can make the scientific experiments feasible,which could provide biomechanical basis for understanding the mechanisms of hip and the degeneration progression of FHN without requiring in vivo studies.Evidence suggests that mechanical factors play an important role in the development and progression of OA[33,34].Hip joint is the largest enarthrosis joint of human;its pathological progression is closely related to the biomechanics[35].Hence,in this study, we adopted computational biomechanical techniques to investigate the effect of enhanced LEA motion on OA progression after FHN.

The purpose of this study was to investigate biomechanical behavior changes of necrotic femoral head during increasing LEA angle,offering scientific basis on choosing the LEA procedure for the exact classifications.In this study,thirty computational biomechanical models were used and the results indicated that the deterioration of FHN into OA is dependent on the extent of STP recovery.We hypothesized that enhanced LEA motions during gait may recover the STP of femoral head and protect against OA progression.The results of this study support our hypothesis:Firstly,as shown in Figure 4,LEA motions could adjust the STPof the femoral head in all conditions.As increasing the LEA angle,STPof type B could be transformed into type A,STPof type C1 could be transformed into type B.STP of type C2 could be transformed into type C1.The lower classification is associated with the lower risk of collapse.Hence,we believe that enhanced LEA motions could reduce the collapse risk of FHN,especially during type A,B and C1,while type C2 of FHN involved surgical intervention may get a better clinical outcome.STP loads principal compressive stress,whose morphology may reflect the function of principal compressive trabeculae.Damage areas of STP gradually reduce in all classifications of FHN as increasing LEA motions.Hence,we believe that enhanced LEA motions may recover the bearing capability of trabeculae.These may due to the reason that the LEA motions moved the principal compressive stress from necrotic region to lateral normal trabeculae.As a result,the cortical bones need not to overload with work to retain the morphology and function of the femoral head.Second,Hadley and Maxian reported that the excessive hip joint contact stress may contribute to the processes of OA[36,37];in our study,as shown in Figure 5,the results reveal that there was no significant difference in contact stress between the four settings of FHN and normal femoral head during LEA motions.These findings provided unique biomechanical insight into treatment results of LEA motion in FHN.Hence,we believed that enhanced LEA motions could not increase the risk of OA.

Conclusion

In summary,the results from our computational biomechanical analysis suggest that enhanced LEA motion can effectively reduce collapse risk and interrupt the disease pathway of FHN deteriorated into OA in the early stages of FHN.Furthermore,our findings indicate that the LEA procedure is suitable for type A,B and C1,while type C2 of FHN involved surgical intervention may get abetter clinical outcome.

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