ANTHROPOMETRIC MEASUREMENTS FOR ENDOMEDULLARY STEM OF REVISION KNEE ARTHROPLASTY

Abstract

Near 70% of failures of knee arthroplastys due to septic or aseptic mobilization are complicated because of massive loss of bone stock. In these cases surgeon have to perform a salvage procedure to restore legamentous balance, articular plane direction and axes of lower limb, finally to fill bone defects. Today intrinsic biomechanical stability of revision implant is entrust to sophisticated design and materials of custom made and modular implants. Endomedullary stem has to assolve specific functions: mechanical stability contrasting stress due to the boneloss, offering support for omoplastic or spongious bone innests in femoralor tibial defects. There are paucity of study in literature about dimension and morphology of endomedullary canal, probably because of variability between periostal andendostal anatomy in each patients, specially age related. This date has conditioned production of several number of revision stem size for all population. This anthropometric study verify presence of a particular regionin femoral and tibial endomidullar canal not dependent from sex, height, morphotype, important for a good press-fit of revision stem.

Morphological date of midfemoral and tibial geometry was assessed in 50 subjects using Axial Computerized Tomography. Eleven (22%) were in men, thirty nine (78%) were in woman, with avarage of 73 years old (from 57 to 85). Exclusion criteria were previous operation at the same limb, deformity, pathology of bone metabolism. The level of sections were at 20, 18, 16 and 12 centimiters from articular plane for femur and at 8, 10, 13, 16 cm for tibia. Preliminar Ct scan with hight resolution program with bone alghoritm was performed. Axial view of any section was visualized at real dimension and maximal and minimal diameter of sections e were measured in millimiters.

Areas of any section is different in each patient; this variability is greater near articular plane. Infact in our sample SD (Standard Deviation) of diameters of proximal femoral and distal tibialsections was lesser than SD of the other measurements. No difference of results about sex was noted. On base of these measuremts more proximal two femoral sections and two more distal tibial sections were considered to elaborate an ideal area for anchorage zone of anatomical revision stem. We subdivided all sample in five groups with homogeneous value of diameter in selected sections. For any groups tangent & #945; of an ideal trunk of cone including maximum and minimum value of diameter considered was computed, and the relative inclination angle. In our sample the greatest diameter in proximal femoral section was 20 mm and the lowest 10 mm; for tibial measuremnts the greatest value in distal tibial level was 18 mm and the lowest was 10 mm. Inclination angle was ever around 2° in all groups. Moreover we have calculated the presumable length of an ideal trunk of cone that includes the minimum and maximum value of femural and tibial diameter measured.

About 5% of knee arthroplastys is destinated to an unsuccessfull. In many cases loosing of bone stock is huge and localized near articular plane. Afterward it’s important to restore biomechanical stability through endomedullary stem of revision implant. There are not many reports about morphology and anatomical study of femoral and tibial endomedullary canal. Many authors focus the attention on bone density or functional axes of the lower limb. Some authors studied remodelling process age-related about periostal ad endostal bone; cortical area undergoing thinning specially over fourthy years old. Our sample had mean of 73 years old: remodelling process is almost complete and then it is a good referenceto extrapolate real data about endomedullary morphology and width. Variability of data in all population about diameter of endomedullarycanal is lesser near femoral and tibial istmo, in particular around 18–20 cm from articular plane for femur and at 13–16 cm for tibia In our opinion short stem can’t guarantee good press-fit because of extreme variability of diameters in the population near knee. This anthropometric study confirm presence of anatomical area of the midfemural and tibial canal with common geometric characteristic in allpatient; it can be used to realize an anatomical stem that guarantees a good contact bone/prosthesis and then an optimal bone integration. On base of our results it is possibile realize few revision stem because in all groups of patients studied at 18–20 cm from knee for femur and at 13–16 cm for tibia, angle of inclination was ever 2°.