The stability and durability of total hip reconstruction is dependent on many factors that include the design and anatomic orientation of prosthetic components. An analysis of femoral component head size and acetabular component orientation shows an interdependency of these variables and joint stability. Increased femoral component head size can increase hip stability by increasing the prosthetic impingement-free range of hip motion and by increasing the inferior head displacement required before hip dislocation. Increasing the femoral head size from 22 mm to 40 mm increases the required displacement for dislocation by about 5 mm with the acetabular component at 45°of abduction; however, increasing acetabular component abduction greatly diminishes this stability advantage of larger femoral heads. Vertical acetabular component orientation and femoral component head subluxation are each predicted to more than double the tensile stress with acetabular component polyethylene compared with components at 45°of abduction. With a desirable acetabular component orientation, the use of larger femoral heads may result in improved joint stability and durable use of polyethylene. With high abduction acetabular component orientation, the use of larger femoral heads contributes little to joint stability and contributes to elevated stress within the polyethylene that may result in implant failure.

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Periacetabular osteolysis is a potentially difficult surgical challenge, which can often drive the choice of reconstruction methods used in revision hip replacement. For smaller defects, impaction of bone grafts may be sufficient, but larger defects can require filler materials that provide structural support in addition to filling a void. This study utilized finite element analysis (FEA) to examine the state of stress in periprosthetic pelvic bone when subjected to a stair-climbing load and in the presence of two simulated defects, to show the effect of implanting a defect repair implant fabricated from Trabecular Metal. Even a small medial bone defect showed a local stress elevation of 4x compared with that seen with an acetabular implant supported by intact periacetabular bone. Local bone stress was much greater (8x the baseline level) for a defect case in which the loss of bone superior to the acetabular implant permitted significant migration. FEA results showed that a repair of the small defect with a Trabecular Metal restrictor lowered periprosthetic bone stress to a level comparable to that in the case of a primary implant. For the larger defect case, the use of a Trabecular Metal augment provides structural stabilization and helps to restore the THR head center. However, stress in the adjacent periprosthetic bone is lower than that observed in the defect-free acetabulum. In the augment case, the load path between the femoral head and the pelvis now passes through the augment as the superior rim of the acetabulum has been replaced. Contact-induced stress in the augment is similar in magnitude to that seen in the superior rim of the baseline case, although the stress pattern in the augment is noticeably different from that in intact bone.

The intramedullary canal presence of a stemmed prosthetic component can be expected to change the distribution of stress from the joint loading to the adjacent skeleton. The reconstructed skeletal stiffening that results for relatively rigid prosthetic components can result in periprosthetic stress shielding. The abrupt change in reconstruction stiffness that can occur at the implant stem terminus can be associated with periprosthetic pain. A stem end “clothes pin” slot, intended to make a stem end more flexible, produces a substantially asymmetric and abrupt alteration of stem stiffness. The present work is directed to analyzing ways that the structural stiffness of the terminus region of a prosthetic stem can be controlled to provide a symmetrical transitional region of controllable load transfer to the surrounding bone. It is hypothesized that through implant design, prosthesis-to-bone interface pressure and periprosthetic bone stress levels at a prosthetic stem terminus can be reduced and that this will be associated with a reduced occurrence and severity of “end of stem pain” in a variety of prosthetic applications.

Osteolysis of the pelvis after total hip replacement (THR) can result in several types of bony defects within or around the acetabulum [1]. These defects are classified into various categories based on the extent and location of the host bone loss [2, 3]. A severe acetabular defect with at least 30% of bone loss and progressive amounts of superior rim deficiencies can be classified as a Paprosky type IIIA cavitary defect [4] (Figure 1-I). A significant amount of superior migration of the cup can be expected as the deficient acetabulum with nonsupportive superior dome will be unable to support an acetabular component at the anatomic hip center without using structural allograft, custom implants or reconstruction cage [4, 5]. A new reconstructive technique (Figure 1-II) uses modular Trabecular Metal™ (TM) augments (Figure 1-III) to fill the acetabular defects at the time of revision THR so that regular hemispheric uncemented acetabular components can be used to allow for the potential of biologic fixation.