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Excellent primary stability of uncemented acetabular shells is essential to obtain successful clinical outcomes. However, in the case of developmental dysplasia of the hip (DDH), aseptic loosening may be induced by instability due to a decrease of the contact area between the acetabular shell and host bone. The aim of this study was to assess the primary stability of two commercially-available acetabular shells, hemispherical and hemielliptical, in normal and DDH models. Synthetic bone was reamed using appropriate surgical reamers for each reaming condition (normal acetabular model). The normal acetabular model was also cut diagonally at 40° to create a dysplasia model. Stability of the acetabular components was evaluated by the lever-out test. In the normal acetabular model conditions, the maximum primary stabilities of hemispherical and hemielliptical shells were observed in the 1-mm under- and 1-mm over-reamed conditions, respectively, and the resulting stabilities were comparable. The lateral defect in the dysplasia model had an adverse effect on the primary stabilities of the two designs. The lever-out moment of the hemielliptical acetabular shell was 1.4 times greater than that of the hemispherical acetabular shell in the dysplasia model. The hemispherical shell is useful for the normal acetabular condition, and the hemielliptical shell for the severe dysplasia condition, in the context of primary stability.

Antibiotic-impregnated hip cement spacers of various types and materials have been used in the treatment of periprosthetic hip infections. We developed a handmade spacer by using polymethylmethacrylate (PMMA) and/or α -tricalcium phosphate ( α -TCP). In this study, we retrospectively reviewed the surgical outcomes in 36 consecutive patients treated with 2-stage revision total hip arthroplasty by using our antibiotic-impregnated hip cement spacers. We aimed to analyze the infection control and reinfection rates after revision surgery. Moreover, we analyzed the possible predictors of postoperative reinfection. After exclusion of 1 patient who died immediately after the first-stage surgery, infection was controlled in 33 of the 36 hips (success rate, 91.7%). Two of these 33 hips underwent resection arthroplasty. Of the 36 hips that had been treated with the antibiotic-cement spacer, 31 hips (86.1%) were eligible for the second-stage prosthesis re-implantation. The 31 protocol hip joints of patients followed up for >6 months (mean, 48.6 months). Ten of these 31 hips (32.3%) became reinfected. No possible predictor examined differed significantly between the reinfection-positive and reinfection-negative groups. However, spacers consisting of PMMA cement alone were associated with the highest risk of reinfection. Therefore, α -TCP-containing antibiotic-impregnated hip cement spacers might decrease the reinfection rate in patients undergoing re-implantation.

Adequate fixation at the time of cementless stem implantation depends on the operator's experience. An objective evaluation method to determine whether the stem has been appropriately implanted may be helpful. We studied the relationship between the hammering sound frequency during stem implantation and internal stress in a femoral model, and evaluated the possible usefulness of hammering sound frequency analysis for preventing intraoperative fracture. Three types of cementless stem (BiCONTACT®, SL-PLUS®, and AI-Hip®) were used. Surgeons performed stem insertion using a procedure similar to that employed in a routine operation. Stress was estimated by finite element analysis, the hammering force was measured, and frequency analysis of hammering sound data obtained using a microphone. Finite element analysis showed a decrease in the hammering sound frequency with an increase in the estimated maximum stress. When a decrease in frequency was observed, adequate hammering had occurred, and the continuation of hammering risked fracture. Based on the relationship between stress and frequency, the evaluation of changes in frequency may be useful for preventing the development of intraoperative fractures. Using our method, when a decrease in frequency is observed, the hammering force should be reduced. Hammering sound frequency analysis may allow the prediction of bone fractures that can be visually confirmed, and may be a useful objective evaluation method for the prevention of intraoperative periprosthetic fractures during stem insertion.

During cementless stem fixation, impaction of the stem is occasionally complicated by bony injuries. Small fractures not visible to the eye during surgery or on post-operative radiographs may remain undetected, and the incidence of such injuries may be underestimated. Employing the same techniques as those employed during total hip arthroplasty, we implanted cementless stems into artificial femora, with equivalent mechanical characteristics to living femora. The hammering force applied to the femur and the displacement of the stem and femur were measured using a load sensor and imaging, respectively. The von Mises stress generated in the femur during cementless stem press-fixation was also measured using finite element analysis. Average hammering force under these conditions (9.25 kN) was sufficient to cause damage to the artificial femur. The first two of eight hammer strikes caused most displacement of the stem. The von Mises stresses generated by the first and second hammer strikes were 31 and 68 MPa, respectively. Applying a high average hammering force to the stem after displacement has ceased probably contributes to the generation of intraoperative fractures during cementless stem fixation. Given that two strikes appeared to be sufficient, we believe that hammering force should be reduced to a micro-adjustment level after the second hit.

Background: Appropriate contour design of a femoral stem is important for the secure primary fixation. Although the relationship between the contour and stress or micro-motion at the fixation site has been analyzed, guidelines on stem design have not been established., Methods: Different kinds of finite element models of three femoral stems were constructed for computer simulation. These models had the contour designs that head for the tight mechanical fixation by the different ways, respectively. Boundary and initial conditions were (i) rigid contact of the distal end of the model femur with the rigid base. (ii) Stepping load of 800 N or 1800 N was applied to the proximal top of the stem. (iii) Stepping load of 640 N or 1440 N was pulled from the greater trochanter of the femur as muscle force. The ratio of displacement to the vertical load, which was defined as fixation stiffness, was calculated., Findings: The mean displacement of the intra-medullary cruciate stem was 27.6 nm. For the VerSys stem with sharp fins, the mean displacement was 32.9 nm. For the PerFix SV stem with a flange, the mean displacement was 52.8 nm. Vertical fixation stiffness estimated on the intra-medullary cruciate stem, the VerSys stem, and the PerFix SV stem was 461, 264, and 313 MN/mm, respectively., Interpretation: Contour design of a femoral stem is important for achieving secure fixation, which prevents loosening. We introduce fixation stiffness as a measure to theoretically evaluate the primary fixation of femoral stems.