We would like to thank Dr. Kwong for his insightful comments regarding minimizing the risk of iatrogenic fracture during total hip arthroplasty. We agree that reamers can be worn or dulled with use, and that preparing an acetabular bed with a particular reamer does not guarantee that the bed is exactly the size labeled on the reamer. Additionally, reaming technique probably can affect the accuracy of bed preparation, certainly a millimeter of reamer "toggle" in one's hand or on the driver shaft can and does occur, probably resulting in beds that are not exactly 53mm in diameter simply because the surgeon used a 53mm reamer.

Dr. Kwong's concept of sizing the prepared bed with fenestrated bipolar trials is a good idea, as visual confirmation of trial seating in the prepared bed can be obtained. Trials made of clear plastic are also available for this step, and these can be useful when using monoblock cups to asses seating depth and bed diameter.

We think that methods to measure the prepared bed will become even more important with the growing enthusiasm for metal on metal hip resurfacing using monoblock metal shells that don't allow visual confirmation of full cup seating. It may be that such monoblock components may best be manufactured as true hemispheres to minimize fracture risk with component seating, but this speculation remains to be substantiated.

Obviously, due to the retropective nature of our review of an enormous number of cases, we cannot comment on the steps taken by individual surgeons to assess the accuracy of reamer to prepared bed diameter. It was not, and is not customary, however, for us to perform additional measurements after reaming. For example, for a particular shell the surgeon would typically under-ream by 1 to 2mm for a true hemishpere and to the labeled size for an elliptical shell. During shell impaction, if excessive force was felt to be necessary to seat the shell, most surgeons would "touch" the rim with a reamer 1mm larger than the last reamer used, and then seat the cup. We think that surgeon judgement and intraoperative assesment of the force needed to seat a cup in a particular patient should guide the need for additional reaming. Also, knowledge of certain cup design features that increase fracture rates is useful.

It should be noted that with this technique, our fracture rates were extremely low for hemispheric and modular ellipitcal shells. One could visibly tell when the shell was fully seated by looking through the screw holes. With monoblock shells, the surgeon assumed by tactile, and probably auditory feedback when to stop impacting the shell. We do feel that the increased frictional coefficient of trabecular metal, the elliptical design, the monoblock design, and the fact that these shells were typically implanted into younger patients with good bone stock all additively contributed to the higher hoop stresses and higher fracture rate noted.

To The Editor:

The article by Dr. Haidukewych regarding “Intraoperative Fracture of the Acetabulum During Primary Total Hip Arthroplasty”(1) calls attention to the critical importance of surgical technique in reducing the risk of intra-operative and post-operative complications. Integral to this is a proper surgeon understanding of the design features unique to each implant as well as the instruments to be used. Our own research supports the practice of trying to achieve a 1-2 mm interference fit as being optimal(2).

What is not reported in this study is the technique utilized at the time of surgery to determine the size of the acetabular bed prepared by reaming. White et al.(3) found a tolerance variation in the actual size of hemispherical acetabular reamers compared to the size etched on the reamer shell. Also, dimensional changes in the reamer occur due to wear of the cutting teeth and may further increase with re-sharpening. Thus,the diameter of the bony bed produced may be smaller in dimension than that marked on the reamer. In this scenario, with the use of the monoblock hemi-ellipsoid acetabular components (Implex/Zimmer)-- which are 2 mm larger in diameter at the mouth of the implant--a 3 mm or greater interference fit could result, increasing the risk of iatrogenic fracture and/or incomplete seating of the prosthesis.

Because of the high surface friction of porous tantalum against bone, a “medializer” was developed 10 years ago for this implant system to reduce the need to generate potentially high insertion forces with this prosthesis. Was this tool utilized at the author's institution during this acetabular component insertion?

Regardless of the use of hemispherical or elliptical design implants, rather than measuring the reamer as described by Dr. Haidukewych, I would recommend using machined aluminum bipolar sizers (made by both Zimmer, Smith and Nephew, and others)--which have radially oriented slots in the dome--to directly visualize the contact with the bone in order to measure the actual size of the acetabular bed as prepared. This information can then be utilized by the surgeon in making a decision with regard to the acetabular component size to be used. Surgeon knowledge as to the actual interference fit obtained is essential in reducing the risk of intra- operative fracture with any device.

The author(s) of this letter to the editor did not receive payment or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author(s) are affiliated or associated.

References:

1. Haidukewych GJ, Jacofsky DJ, Hanssen AD, Lewallen DG. Intraoperative Fractures of the Acetabulum During Primary Total Hip Arthroplasty. J Bone Joint Surg Am. 2006; 88: 1952-1956.

2. Kwong LM, O'Connor DO, Sedlacek RC, Krushell RJ, Maloney WJ, Harris WH. A quantitative in vitro assessment of fit and screw fixation on the stability of a cementless hemispherical acetabular component. J Arthroplasty. 1994;9(2):163-70.

3. White RE et al. Effect of Prosthesis and Instrument Manufacturing Tolerance on Surgical Technique of the Bone Ingrowth Acetabulum. 61st Annual Meeting of the American Academy of Orthopaedic Surgeons. February 26, 1994.