Effects of early and late stage cement intrusion into cancellous bone

Femoral Stress Changes after Total Hip Arthroplasty with the Ribbed Prosthesis: A Finite Element Analysis

Background

A total hip reconstruction is related to the stress distribution throughout the prosthesis, cement, and femur. Researches on reducing the stress in all components to minimize the risk of failure are of great significance. The objective of our study was to determine the biomechanical variation in overall femoral stress and periprosthetic femoral stress distribution after implantation with the Ribbed anatomic prosthesis.

Methods

Three-dimensional finite element models of intact femur and Ribbed prosthesis were developed according to the morphology, while the hip joint loading and the strength of related muscles were applied in the models. The overall stress changes of the intact femur before and after the implantation were analyzed, and the periprosthetic stress distribution especially in the proximal region of the femur was quantified.

Results

As a result, the overall stress pattern of the femur did not change after the implantation compared with the intact femur. The region of peak stress value was located in the middle and lower segments of the full length femur, but the stress value level decreased. The final prosthesis resulted in a significant decrease in the equivalent stress level of the periprosthetic bone tissue, and the most severe area appeared at the endmost posterior quadrant. The stress shielding ratio of the Ribbed prosthesis was 71.6%. The stress value level gradually increased towards the distal part of the prosthesis and recovered to physiological level at the end of the prosthesis.

Conclusions

The Ribbed prosthesis can cause significant stress shielding effect in the proximal femur. These results may help optimize prosthetic design to reduce stress shielding effect and improve clinical outcomes.

The modern, hybrid total hip arthroplasty for primary osteoarthritis at the Hospital for Special Surgery

We describe our technique and rationale using hybrid fixation for primary total hip arthroplasty (THA) at the Hospital for Special Surgery. Modern uncemented acetabular components have few screw holes, or no holes, polished inner surfaces, improved locking mechanisms, and maximised thickness and shell-liner conformity. Uncemented sockets can be combined with highly cross-linked polyethylene liners, which have demonstrated very low wear and osteolysis rates after ten to 15 years of implantation. The results of cement fixation with a smooth or polished surface finished stem have been excellent, virtually eliminating complications seen with cementless fixation like peri-operative femoral fractures and thigh pain. Although mid-term results of modern cementless stems are encouraging, the long-term data do not show reduced revision rates for cementless stems compared with cemented smooth stems. In this paper we review the conduct of a hybrid THA, with emphasis on pre-operative planning, surgical technique, hypotensive epidural anaesthesia, and intra-operative physiology.

Enhancing proximal femoral pressure during cemented stem insertion

BACKGROUND

Poor proximal femoral pressures during a cemented primary hip replacement may compromise proximal fixation and cause early aseptic loosening. Occlusion of the calcar, during stem insertion, achieves a uniform and sustained rise in the proximal intra-medullary pressure, which should enhance cement penetration into bone and improve long-term survival of the prosthesis.

METHODS

54 Exeter stems were cemented into femoral moulds prepared from plaster of Paris. Pressure transducers were connected to the proximal, middle and distal zones of the mould. After cement insertion, stems were implanted with occlusion of the calcar by either nothing, a thumb placed medially or an Exeter horse-collar. 18 stems each were inserted into Palacos-R at 3½-4 minutes and 4-4½ minutes after mixing and Simplex-P 6-6½ minutes after mixing and the intra-medullary pressures were measured. Data were subjected to regression analysis using SPSS.

RESULTS

Proximal and distal intra-medullary pressures were significantly higher (P < 0.01-0.001) with proximal occlusion in all cements. The highest pressures were achieved with Palacos-R at 4-4½ minutes after mixing, with proximal thumb occlusion. Stem insertion into Palacos-R at 3½-4 min. or 4-4½ min. after mixing, gave higher pressures than into Simplex-P regardless of the method of occlusion. With Simplex-P, there was a trend to higher proximal intra-medullary pressures with the horse-collar.

CONCLUSION

Occluding the calcar during stem insertion into cement achieves and sustains high intra-medullary pressures in both the proximal and distal femur. The highest pressures are obtained with stem insertion into Palacos-R at 4-4½ minutes after mixing, with proximal thumb occlusion. A horse-collar achieves slightly higher pressures with Simplex-P.

Timing of femoral prosthesis insertion during cemented arthroplasty: cement curing and static mechanical strength in an in vivo model

BACKGROUND

Modern cementing techniques aim to fix the implanted femoral prosthesis in the medullary cavity to minimize long-term complications such as aseptic loosening. The cure stage of bone cement into which the femoral component is being inserted is an important variable that is decided at the time of surgery. Late-cure cement is more viscous than early-cure cement and requires greater force on the part of the surgeon to insert the femoral prosthesis. We compared 2 cementing techniques, femoral component insertion into early-cure cement and insertion into late-cure cement, using an in vivo model to identify if cement cure stage affects the strength of the bone-cement interface.

METHODS

We performed bilateral hemiarthroplasties using only the femoral component in vivo on paired porcine femora. The femora were harvested and cross-sectioned in preparation for strength testing. We measured bond strength by peak load required to push the femoral prosthesis and surrounding cement mantle free of the cancellous bone.

RESULTS

All radiographs showed good cement interdigitation with no evidence of radiolucent lines at the bone-cement interface. We could not differentiate the early-cure and late-cure groups on postoperative radiographs. The mean failure load for the late-cure arthroplasties was 908 N (standard deviation [SD] 420 N), whereas the mean failure load for the conjugate early-cure arthroplasties was 503 N (SD 342 N).

CONCLUSION

Femoral component insertion into late-cure cement required significantly higher loads for push-out than femoral component insertion into early-cure cement.

The effect of cement mixing time on the biomechanics of cement augmented plated fractures in canine femora

OBJECTIVE

The goal of this study was to determine the effect of cement mixing time and, hence, cement viscosity on the biomechanical behavior of femoral fracture fixation.

DESIGN

Cadaveric plated canine femoral fracture model, comparing treatments in matched pairs.

SETTING

Orthopaedic biomechanics laboratory.

INTERVENTION

Cement was inserted both as a liquid and as a paste in standard and oversized screw holes to augment fixation with plates and screws.

MAIN OUTCOME MEASUREMENTS

Standard 4-point bending tests were performed to obtain stiffness and failure load values.

RESULTS

Liquid cement had a 1.38 times increase in stiffness and a failure load 1.84 times greater compared with paste cement, regardless of hole size with a gap at the fracture site (P < 0.05). Liquid cement had a force to failure of 1.77 and 1.91 times in the standard-sized and oversized holes, respectively, when compared with paste cement (P < 0.05).

CONCLUSIONS

When the cement was inserted in a liquid state in a plated femoral diaphyseal fracture with a gap, screw purchase augmentation achieved greater bending stiffness and resisted a greater failure load.

Factors affecting the cement penetration of a hip resurfacing implant: an in vitro study

An understanding of how the cementing technique influences cement penetration is important for surgeons to plan and conduct hip resurfacing procedures. Therefore the aim of this study is to determine the influence of the following parameters on cement penetration: use of pulse lavage, type of cement, and the standing period of the cement. Nine fresh frozen paired whole cadaver femora were used to investigate cement penetration. The femora were divided into three paired groups: (A) compared the use of pulse lavage to no pulse lavage, (B) compared two low viscosity bone cements SULCEM-3 (Zimmer GmbH, Switzerland) and Surgical Simplex (Stryker Orthopaedics, USA), and (C) compared bone cement viscosities using standing times of 1.5 min. and 3 min. The Durom Hip Resurfacing implant (Zimmer GmbH, Switzerland) was used in all groups. A single slice was taken out of the center of each head. Cement penetration ratio (penetration area divided by the bone area enclosed by implant) and mean cement penetration depth were measured. Cement penetration is increased using pulse lavage, and has the tendency to increase with increasing the cement standing time from 1.5 to 3 min. No difference in cement penetration was found when different cement brands of comparable viscosity were used.

New technology in cemented total hip arthroplasty

The long-term results of total hip arthroplasty (THA) are predicated by excellent surgical techniques. New technology offers the hope of improving outcomes by providing to surgeons tools that make surgical procedures predictable. Techniques that improve the bone-cement-prosthesis composite should enhance long-term fixation. Less invasive surgical techniques that allow rapid recovery from THA have been recently described. Image-guided surgery may enable surgeons to accurately reconstruct the arthritic hip and improve outcomes.

In vitro analysis of the cement mantle of femoral hip implants: development and validation of a CT-scan based measurement tool

We developed, validated and assessed inter- and intraobserver reliability of a CT-scan based measurement tool to evaluate morphological characteristics of the bone-cement-stem complex of hip implants in cadaver femurs. Two different models were investigated: the stem-cavity model using a double tapered polished femoral-stem that is removed after cement curing and the plastic-replica model using a stereolithographic stem replica that is left in place during CT-scanning. Software was developed to segment and analyze connective CT-images and identify the contours of bone, cement, and stem based on their respective gray values. Volume parameters (whole specimen, cement, stem, air contents of bone and cement), concentricity parameters (distances between centroids of stem and cement, cement and bone, stem and bone), contact surfaces (bone/air and cement/bone) and bone cement mantle thickness parameters were calculated. A three-dimensional protocol was developed to evaluate the minimal mantle thickness out of the CT-plane. The average accuracy for surfaces within CT-images was 7.47 mm2 (1.80%), for bone and cement mantle thickness it was 0.51 mm (9.39%), for distances between centroids it was 0.38 mm (18.5%) and contours: 0.27 mm (2.57%). The intra- and interobserver reliability of air content in bone and cement was sub-optimal (intraclass-correlation coefficient (ICC) as low as 0.54 with an average ICC of 0.85). All other variables were reliable (ICC>0.81, average ICC: 0.96). This in vitro technique can assess characteristics of cement mantles produced by different cementing techniques, stem types or centralizers.