Hip Resurfacing, Osteoarthritis Treatment

1/15/08

Significance of Hip Resurfacing Design Differences - Conserve®Plus Hip Resurfacing vs. other hip resurfacing systems.

Harlan C. Amstutz

Component Size Increments

Femoral and acetabular components come in 2mm increments compared to 4 mm with the BHR and Cormet which enables the surgeon maximum flexibility to optimize bone preparation for different anatomical and pathologic conditions facilitating minimal bone removal of the acetabulum and/or femoral head. This is especially helpful when the head neck ratio is low or the acetabular walls are thin. A larger femoral surface area potentially enhances long-term durability.

Femoral Design and Bone Preparation

The Conserve®Plus femoral bone instrumentation and preparation technique permit fine-tuning and optimizing the pin placement based on the anatomy and pathology of the femoral head and neck, up to and including the final reaming, dome cut off and subsequent hole preparation for the stem. This feature allows the surgeon to carefully examine the head and neck as it is being reamed and downsized to take advantage of the superior and/or inferior subcapital recess when present, to optimize bone preparation and to avoid notching by making subtle angle changes in all directions. This ability to change with the Conserve®Plus is a major difference compared to other resurfacing systems which require optimizing the pin placement prior to any reaming because the stem hole is made first before the reaming begins and dictates the subsequent component orientation. With none of the other systems is it possible to subsequently alter the reaming direction to make changes based on anatomy and pathology, which often only become apparent as the reaming progresses.

Cement Mantel Thickness

The designs such as the Conserve®Plus which allow for a one mm cement mantle facilitate a more uniform pressurization and penetration of doughy acrylic with hand pressure into all of the reamed areas of bone and facilitate full component seating with pressure or light mallet taps. The systems such as the BHR and Cormet which have tighter and almost press-fit femoral components to the prepared bone permit only a very thin or no cement mantle and the surgeon must use cement in a low viscosity state because there is no exit strategy for the cement in a more viscous state. The low viscosity cement cannot be hand pressurized into the cylindrical reamed bone, diminishing penetration. Only a minimal amount of acrylic will penetrate into the cylindrical reamed bone because of the shear force generated by the component as it is inserted. In addition, because of the tight fit and the difficulty of extruding the cement between the component and the bone, there is a danger of over penetration of cement into the proximal femoral head, which has been associated with some failures. Further, full seating of the component is more difficult, often requiring hammer blows which could risk injury to the femoral neck... Long-term follow-up studies will be needed to determine the significance and effects on survivorship of these differing designs and techniques but the results of our techniques have shown that short and mid-term complication-free results without either neck fracture or loosening are possible in hips resurfaced with risk factors. Since comparable results have not been demonstrated with other systems, this validates the design principles of the ‘Conserve®Plus System.

Socket Design

A) Profile

The normal acetabulum is less than a hemisphere. To minimize bone removal of the acetabulum it is desired that the socket coverage be less than 180 degrees. A low profile socket 165°-170° vs. 180° ( Conserve®Plus vs. BHR and Cormet ) facilitates a more anatomic and conservative approach. Reaming to the floor of the acetabular fossa is generally not necessary. All but one of the designs are spherical. Eccentric sockets with an additional 2mm at the equator may make socket seating more difficult. Accurate seating of the socket is dependent on instrumentation available with the Conserve®Plus system to properly evaluate the acetabular cavity size and sphericity.

B) Socket Fixation

The plasma spray surfaces and especially small porous beads (Conserve®Plus) facilitate optimal initial socket stability in comparison to those sockets (BHR and Cormet)with much larger macro beads which require fins and careful technique to avoid a socket spin-out. Because of excellent initial stability with the Conserve®Plus it is feasible to leave the socket uncovered in cases with acetabular dysplasia ~ 1 to 2cm laterally and/or posteriorly and adjuvant fixation is unnecessary. Pelvic adjunct screws are necessary with the BHR and Cormet Designs in Crowe class II DDH cases. More over, those systems which have a simple spherical geometry such as the Conserve®Plus without adjunct fixation are more simply extracted and reinserted at surgery to optimize component positioning at surgery or removed with minimal bone loss should they ever have to be removed later after ingrowth.

Wear

There is insufficient data at this time from clinical retrievals to compare the in-service wear of the differing designs This will be difficult because of the multifactorial nature of wear and there are few comparative simulator studies with all the variables controlled. Wear can be minimized by optimizing the clearance and roundness tolerances, and the surface finish of the components to facilitate lubrication. Contrary to what one manufacturer professes, simulator studies have established that when the conditions were similar to implanted hips and where size and clearances were similar between different manufacturing processes, there is no difference in the wear whether the components are made from as cast Cobalt-chrome or heat treated and solution annealed or forged. However it is possible to reduce wear by incorporating a differential hardness between ball and socket, forging the femoral component to make it harder as has been done for the Conserve®Plus system of implants by WMT. This “differential hardness” potentially reduces the adhesive wear and also improves the abrasive wear of the side most susceptible to wear the femoral head in case of the hip articulation. Clearance allows for lubrication and is size dependent. When the clearances are large( 220+) such as with the BHR and the Cormet there is a potential for more wear especially when the socket is oriented in a more vertical position(>55°). Thus the tolerances for both clearance and roundness are very important.

Stem Design

The importance of size and shape of the stem is yet to be determined. The Conserve®Plus metaphyseal stem is tapered and thinner than with the BHR and Cormet designs and the length is proportional to the size of the component. It occupies a smaller portion of the femoral neck, which may decrease the potential for stress shielding in the femoral head and neck, especially in patients with small femoral head size. This may be a factor explaining the higher incidence of neck narrowing in the designs which have larger diameter stems (BHR and Cormet and in cementless applications by Cormet) compared to the Conserve®Plus. Long-term analysis will be required to evaluate the true significance of these differences.

Comparative table of the features and characteristics of the currently available metal-on-metal hip resurfacing designs.

System	Date	Process	Heat treatment	Clearance (?m)	Coverage (o)	Shape	Surface	Shell thickness
C+	1996	Wrought (Fem) Cast (Acet)	HIP, SHT	100 to 220	170	Truncated hemisphere	Sintered Co-Cr beads (50-150?m) +/- HA	3.5 or 5
BHR	1997	Cast	None	260 (Size 54 only)	160 in, 180 out	Hemisphere	Co-Cr beads (0.9 - 1.3mm) cast-in + HA	3 Rim, 6 Dom
CORMET	1997	Cast	HIP, SHT	250	180	Equatorial expansion	Ti-VPS + HA	3 and 4 (2 cups per head)
DUROM	2001	Wrought	N/A	150	165	Truncated hemisphere	Ti-VPS	4
ASR	2003	Wrought	HIP, SHT	100	156 in 170 Out	Truncated hemisphere	Sintered Co-Cr beads (200-300?m) + HA	3 - 5
ReCap	2004	Cast	None	?	180	Hemisphere	Ti-VPS +/- HA	3