Cemented Femoral Fixation

Henry W. Hamilton, M.D., F.R.C.S.(C)

Orthopaedic Surgeon

Port Arthur Clinic

Thunder Bay, Ontario

In 1995, Murray posed the question, “Which Primary Total Hip Replacement?” JBJS 77B, 520-527, 1995.

He noted that 90% of implants are now modular.

The only advantage of modularity in a primary femoral prosthesis is the ability to use a ceramic head.

The disadvantages, another particle generating interface and possible disassembly, are obvious.

Murray warned that clinical results not published in peer- reviewed journals must be interpreted with extreme caution, e.g. one manufacturer’s advertising claimed, “the unaltered stem design has been successfully used in thousands of implantations since 1985 with no report of aseptic loosening”. It was noted at the British Hip Society in 1994 that this was not true.

Murray recommended that, “If clinical results are not available, a new implant should only be used if it is included in a properly conducted clinical trial”.

“Improved fixation of the femoral component after THR using a methacrylate intramedullary plug”

Oh, Carlson, Tomford, & Harris. JBJS 60A, 608-613, 1978.

They described in vitro experiments with cadaver femora, and introduced the concept of a distal femoral canal plug and retrograde filling with a cement gun.

They recorded the interface peak pressures with two-thumb packing, after filling the canal with cement, with and without a distal femoral canal plug. No statistically significant differences were found.

However distal peak pressures during the introduction of a tapered stem were 119N/sq cm with a plug, and 49N/sq cm without a plug.

These findings were statistically significant.

These peak pressures occurred during the 10 seconds taken to insert the stem, and fell to zero by 12 seconds.

It was not until 10 years later that Shelley & Wroblewski described experiments, which took into account the back pressure of the intra-osseous blood. Because of its viscoelastic nature, cement pressurised into cancellous bone for 10 seconds, will be extruded by the back pressure of the intraosseous blood as soon as the pressurisation stops.

“The femoral cement compactor”

Oh, Bourne & Harris. JBJS 65A, 1335-1338, 1983.

The 1978 Boston experiments had improved distal femoral canal pressurisation but had done nothing to improve proximal pressurisation that is far more important. This 1983 study addressed this problem by introducing a proximal seal.

Proximal peak pressures, with a distal plug and retrograde filling, were 51N/sq cm using a compactor, compared to 31N/sq cm with digital packing. The difference was statistically significant.

The magnitude and duration of the proximal pressurisation was 32N/sq cm for 15-20 seconds with the compactor, compared to 17N/sq cm for 5 seconds with digital packing.

This was still 5 years before Shelley & Wroblewski experiments were published. Blood back pressure was not considered, and we now know that as soon as the pressurisation is stopped, the cement is extruded from the cancellous bone.

Pressurisation must be continued throughout cement polymerisation.

“Relationship of acetabular wear to osteolysis and loosening in THA”

David Sochart. CORR 363, 135-150, 1999.

235 LFAs in 163 patients, 48 male & 115 female.

All patients were under 40 years old, average age 31.7 years.

The diagnoses were ankylosing spondylitis, CDH, OA, & RA.

Septic cases were excluded.

Follow up was from 6-30 years, average 19.5 years.

Definition of femoral failure:

Loose (demarcation or osteolysis in 3 or more zones).

Stem fracture.

Revision.

17% femoral prostheses failed.

25 year femoral prosthesis survival to failure with:

PE rate of wear < 0.1mm/year 88%

PE rate of wear > 0.2mm/year 0%

A marked variability in the rates of wear was noted.

There was a highly significant relationship between cup wear and component failure and revision.

“A tribological study of retrieved hip prostheses”

Isaac, Wroblewski, Atkinson, & Dowson. CORR 276, 115-125, 1992.

100 Charnley cemented cups, 78 of which had associated femoral prostheses, retrieved at revision surgery for mechanical problems.

54% of the failures were associated with stem loosening.

The roughness of the explanted Charnley femoral heads exceeded the LFA standard of Ra 0.025 microns in 76% cases, and exceeded the International standard of Ra 0.05 in 33% cases.

The penetration of the heads into the cup PE was 0.2 - 4.3 mm.

The femoral head tunnels a hole equal to its diameter into the PE.

The volume of wear is pi x radius squared x penetration.

The volume of PE removed by wear is proportional to the load applied, the sliding distance of the articular surfaces, and the roughness of the steel bearing surface.

The best correlation coefficient is between the service life and the penetration rate.

Failure is, however, multifactorial.

Patient selection: age, weight, activity, and compliance.

Surgical technique: effective pressurisation throughout cement polymerisation, no "bottoming out", cement particles left in the wound, a scratched femoral head.

Penetration and wear: PE wear particles cause osteolysis.

PE wear causes the femoral neck to impinge on the rim of the cup.

Isaac et al suggest changing the bearing surface of the femoral head.

Zirconia Ra 0.005 microns has a harder smoother surface than stainless steel Ra 0.025 and is less likely to be scratched by “third bodies”.

“The influence of scratches to metallic counterfaces on the wear of UHMWPE”

Fisher, Firkins, Reeves, & Hailey. Proc Instn Engrs Vol 209, Part H, J. of Engineering in Medicine, 263-264, 1995.

GUR412 compression moulded, non-irradiated PE pins slid against 316L stainless steel lapped to Ra 0.01 microns. Transverse scratches 2 microns deep with a 1 micron lip were made on the steel counterface. After 200km reciprocating motion, equivalent to 10 years wear, one scratch increased the rate of PE wear 70 times.

“Ceramic bearing surfaces in total artificial joints; resistance to third body wear damage from bone cement particles”

Cooper, Dowson, Fisher, & Jobbins. Journal of Medical Engineering & Technology, Vol 15, No 2, 63-67, 1991.

Because the wear rate of PE is proportional to the surface roughness of the counterface and the considerable variation in the wear rates of PE cups, it has been suggested that radiopaque additives to cement may scratch the surface of femoral heads.

Sliding wear tests were carried out using PE pins loaded onto rotating ceramic & stainless steel discs:

Using a deionized water lubricant.

Lubricant plus particles of barium sulphate 5 –500 microns.

Lubricant plus finely ground zirconia.

Cooper et al concluded that:

Radiopaque additives increase the rate of PE wear.

Ceramic counterfaces are not damaged by barium sulphate.

Zirconia causes less damage to ceramic than to steel.

“A comparative study of the performance of metallic & ceramic head components in THR joint”

Dowson, Wear, 190, 171-183, 1995.

This is a superb review of the tribological characteristics of metallic & ceramic on PE articulations.

Good agreement was found between laboratory and clinical data.

Ceramic heads penetrated into PE cups at a rate 50% less than comparable metallic heads.

I had intended to comment on two excellent papers, “Cemented femoral component surface finish mechanics” Crowninshield & “Migration, stem shape, and surface finish in cemented THA” Huiskes, presented to the Hip Society and now published in CORR, but as one of the authors is here this seemed redundant.

I would like to finish by posing a question pertaining to the precoating of femoral stems.

Is long term bonding possible between stainless steel with an elastic modulus of 200GPa and cement with an elastic modulus of 2.2GPa, when the bond is subjected to cyclical loading which alters their geometry, and periods of rest which allows recovery of creep deformation of the cement polymer?

Charnley was not the first to use PMMA, but he was the first to use it successfully. He used it in bulk, knowing it was strong in compression, and weak in tension or shear. He used it as a grout to transfer load from the prosthesis to bone. Fixation is by mechanical interlock and not by chemical bonding like a glue.

Previous Lecture Index Next Lecture