My initial experience with the anterior hip replacement approach utilized what is known as the Corail Stem. The Corail was developed in Annecy, France by a talented group of physicians. It began use in 1986. Dr. Matta who popularized the anterior approach had developed instrumentation specific to the anterior approach. The learning curve, or how long it takes a surgeon to become proficient in this approach can vary, but it is not short. There are numerous tricks and techniques such as those that allow exposure of the thigh bone and the hip socket in various types of anatomy and sizes and builds of patients. However, as I became more and more comfortable with the technique in approach, I began noticing that there were aspects of the Corail stem that could be improved upon. After long discussions with industry and their biomedical engineers, the decision was made to design a new hip system.
There were four main areas that needed attention, but the overriding goal was not to change the stem where it was proving so successful, from what is known as the “collar” down. The Corail stem had a flawless track record of absolutely no causation of thigh pain, nearly no cases of loosening and incredible patient satisfaction.
Problem number one: Proportionality
As one can imagine, patients come in all shapes and sizes. It makes sense then, that patient’s femoral necks or the upper part of the femur come in all sizes, as well. The current system had only one femoral neck size. This causes problems for all patients outside of a limited size profile. For instance, a small patient would require an excessive amount of bone on the top of the femur being resected, to not err in length of the leg. This resection was detrimental, but unavoidable. The new stem needed proportionality. My goal was smaller sized persons would get a shorter femoral neck, medium sized persons, a medium length femoral neck and large sizes, a longer femoral neck. Proportionality allowed for preserving bone even in the smallest of patients.
Problem number two: Stepping
Imagine walking up a set of stairs and the first four steps have the same seven inch rise. But the next step, without notice, has a 14 inch rise. Imagine trying to tighten a Hex Screw and your Allen Wrench set has an eight and a 10, but no nine – and that is the size called for. The Corail stem had a stepping problem. The new stem would have consistent stepping. If the 11 was a little small or loose, the size 12 would be just right.
Problem number three: Exact correlation in rasp/implant size matching
Once the top of the thigh bone is exposed, rasps or internal shaping devices with increasing sizes are used to cut, compress and prepare the inside of the femur bone for the implant. The surgeon knows when to stop increasing the size of the rasp when he achieves what is called axial and rotational stability. There is an actual change in the sound of the tapping when the rasp is seated properly. This change of sound is because the rasp has now made enough contact with hard cortical bone and prepared the appropriate bed for the implant. However, a frequent finding was that when the appropriate size stem was opened, it would sit proud or was too big relative to the bed which had just been made. The challenge to the engineers was to make the cutting rasp exact, perfectly mated with the corresponding implant.
Problem number four: The old stems were just too long.
The anterior approach requires elevating the femur bone high enough out of the wound to access the internal anatomy of the femur through the top of the wound. In large or muscular patients this is somewhat difficult. It requires more dissection. Modern stems have gotten around this difficulty somewhat by shortening the length of the femoral stem. Correspondingly, as the final touch to the new stem design, all sizes would be shortened 1 cm. This was the least scientific of the changes and was only made after thorough radiographic evaluation indicated that the distal or tip of the stem rarely, if ever, had contact with cortical bone.
In 2017, the first ENTRADA Hip Replacement stem was implanted in Ogden, Utah. All patients, regardless of their size, would now get a stem personally corresponding to their femoral neck anatomy. The preparation tools were exact to meet the implanted stem. The slightly shorter design allowed for maintaining smaller incisions and more gentle care of the soft tissues during the hip replacement. The Entrada stem, having changed next to nothing from the collar down experienced rapid growth, thanks to continued high patient satisfaction and desirable outcomes. The Entrada stem is the nation-wide, highest selling hip stem in the Ortho Development Corporation line.