New Materials for Musculoskeletal RepairBernice Aboud - DePuy - R & D.
DuPuy as a company is 113 years old. It was the first manufacturer of orthopedics in the world, and remains located in Warsaw, Indiana where it had its roots. It held patents for metal splints as early as 1901. The founder of its chief competitor Zimmer, was originally working for DePuy, and out of Zimmer came the third major producer of orthopedics, Biomet. In 1998 DePuy was purchased by Johnson and Johnson. In the hip and knee markets, DePuy and Zimmer are the two most significant producers of replacement orthopedics.
This evening we will discuss three Clinical needs recently filled by research as DePuy R&D. These are:
1) New Polyethylene for Bearings surfaces
2) Stronger Femoral Stems for High Demand Patients
3) Coating innovation for Immediate Fixation with bone mass.
The market for orthopedics is currently seeing segmentation with different needs. For example a low cost limited-life device might suitable for a geriatrics patient just requiring the ability to get-up for self maintenance. Devices with very short post-operative rehab times are required for aging baby-boomers still in the labor force or enjoying active retirement. In the area of Sports medicine, devices promoting regeneration of natural bone and tissue are emphasized.
In the first area (New Polyethylene for Bearing surfaces) there is a need to reduce osteolysis. It was found that oxides present in a layer of the polyethylene are absorbed by bone and can eventually lead to complications. A non-oxidized liner material would be better. Conventional polyethylene liners are Gamma sterilized in air. This leads to the subsurface oxidation layer which can cause the complications. Trials of alternate Gamma sterilization in vacuum and Gamma sterilization in gas plasma procedures were evaluated. A test was devised loading the bearing with 3 time body weight 60 degrees of flexation and 5mm of anterior/posterior motion. With 10 MRad of gas plasma vacuum radiation applied to the polyethylene a 90% wear reduction was achieved. For metal-on-metal articulations a high carbon forged Co-Cr-Mo liner and a high carbon consolidated Co-Cr-Mo head with a clearance of 100 micron was found optimal.
The second area to be discussed is a problem with femoral hip failure in high demand patients. The root of this problem is notch sensitivity in the hip stems. A porous coating is desirable to achieve fixation which is achieved by sintering, but this same sintering reduces the fatigue strength from 65 ksi down to about 45 ksi which can lead to early fatigue failure (rotating fatigue tests). The solution found for this is to re-process the Co-Cr-Mo stem material to start from a homogeneous powder (rather than a casting). The powder is hot isostaticly pressed, and then rolled to form an ingot. Many forging houses could not work with this material, but one "Stealth Engineering" in Michigan has been successful at forging this material. With this processing it has been found that the rotating fatigue of the stems is up by 60% over the former process, and even after application of the porous coating remains at 60 ksi, a vast improvement over the 45 ksi of the former process.
The third and final area to be discussed is a coating innovation to achieve faster (immediate) fixation of the implant to bone. The objectives were to gain greater surface roughness of friction, greater initial fixation, maintain stability under load, torque, and cantilever forces, have greater volume porosity and pore size, and most importantly maintain the clinical heritage of the highly successful Porocoat® coating currently in use.The result of this effort is a 3-d modified coating similar to Porocoat® called Gription®. Standard Porocoat® is a coating made of spherical particles which is ideally applied to a 45% volume porosity level in the titanium implant surface. With Gription® the outer three layers of this are modified to be highly acicular, transitioning to the spherical Porocoat® below that depth. Greater bone attachment has been observed with this strategy and levels of up to 63% vol porosity (vs. 45%) have been achieved. Currently Gription® has only been developed for titanium implant systems. A Gription® equivalent is needed for Co-Cr-Mo.
Estimates are that in 2008 there will be over 700,000 knee and hip precedures and these will increase to over 750,000 by 2012 with much of the increase attributed to the baby-boomer polulation and increased life spans.
Finally several questions were fielded, including the recalls of Zirconia Femoral heads, amount of research effort in orthopedics, and the need for reduced costs.