MECHANICAL CONSEQUENCES OF BONE INGROWTH IN A HIP-PROSTHESIS INSERTEDWITHOUT CEMENT

Long-term biomechanical problems associated with the use of sintered p orous coating on prosthetic femoral stems inserted without cement incl ude proximal loss of bone and a risk of fatigue fracture of the prosth esis. We sought to identify groups of patients in whom these problems are accentuated and in whom the use of porous coating may thus jeopard ize the success of the arthroplasty. We attempted to develop clinical guidelines for the use of sintered porous coating by investigating the long-term biomechanical: effects of bone growth into partially (two-t hirds) porous-coated anatomic medullary locking hip prostheses that fi t well. More specifically, we used a detailed finite element analysis and a composite beam theory to determine tile dependence of proximal l oading of the bone and maximum stresses on the stem on the development of clinically observed patterns of bone ingrowth and the dependence o f the risk of fatigue fracture of the stem on the diameter of the stem , the diameter of the periosteal bone, and the material from which the prosthesis was made. We found that bone ingrowth per se substantially reduced proximal loading of the bone. With typical bone ingrowth, axi al and torsional loads acting on the proximal end of the bone were red uced as much as twofold compared with when there was no ingrowth; bend ing loads on the proximal end of the bone were also reduced. The risk of fatigue fracture of the stem was insensitive to the development of bone ingrowth. However, the risk of fatigue fracture of the stem incre ased with decreased diameters of the stem and the periosteal bone and with increased modulus of the stem. The maximum risk of fracture was f ound in active patients in whom a cobalt-chromium-alloy stem with a sm all diameter had been implanted in a bone with a small diameter. CLINI CAL RELEVANCE: The stabilizing effects of sintered porous coating may not justify its use in patients in whom the coating introduces problem s such as excessive loss of bone and a risk of fatigue fracture of the prosthesis, Our results provide guidelines for the selection of patie nts for the use of sintered porous coating with anatomic medullary loc king devices inserted without cement, First, to minimize the proximal loss of bone, we do not believe that porous coating of any type should be used distal to the lesser trochanter when the patient's life expec tancy is longer than twenty to twenty-five years after the primary rec onstruction procedure, particularly when a stem with a large diameter is to be inserted into a bone with a small diameter. Second, to preven t fatigue fracture of the prosthesis, sintered porous coating should n ot be used anywhere on a cobalt-chromium-alloy stem that is less than approximately eleven millimeters in diameter and is to be inserted in an active or heavy patient in whom the periosteal bone is less than ap proximately twenty-three millimeters in diameter. While insertion of a larger stem into a larger bone may be associated with a risk of fract ure of the prosthesis, depending on the level of activity and the body weight of the patient, these constraints can be relaxed if a sintered titanium-alloy prosthesis is used. Taken together, these findings sug gest that anatomic medullary locking prostheses with sintered porous c oating are best suited for patients in whom a mid-sized stem will be i mplanted in a bone with a larger than average diameter.