The primary reason for the early failure of the Durom Cup implants has been the failure of bone from the pelvis socket to grow into the back side surface of the cup after it has been implanted. Historically, total hip replacements of various brands included a cup that would be attached to the pelvic socket by one or two screws to secure the cup, and some would be cemented into the pelvic socket. But the Durom Cup does not involve any form of cementing or screw device to secure it. The Durom Cup design depended on bone in-growth to secure it.
The failure of bone in-growth into the back side of the cup allows for cup loosening and early device failure, requiring revision surgery. Most patients will experience severe pain with ordinary activities. They may feel pain or a pinching in their groin or in their thigh. They will have difficulty walking, climbing stairs, bicyling, lifting and carrying heavy objects, stooping, crouching, kneeling, getting up from a chair, and getting into and out of a car.
Caution: If you have a Durom Cup implant and you have not yet undergone a "revision" surgical procedure to replace the cup, we strongly advise that you ask your surgeon to preserve the cup to give it to you following the surgery. It should be placed in a sealed container by your doctor or hospital surgical staff. You should keep it in a safe place until you can deliver it to a qualified attorney to represent you in a claim against Zimmer Holdings. At Toney Law Offices, we have the preserved cup examined and studied by a forensic scientist who is familiar with the Durom Cup for the purpose of making a claim against Zimmer Holdings. Preservation of the cup is important for the purpose of proving that the design by Zimmer failed.
Hip Anatomy and Arthroplasty
The hip joint is comprised of two parts: a ball and a socket. The ball is at the top of the thighbone (femur) which fits into a cup or socket-like structure (acetabulum) within the pelvis (Figure 1). The natural hip joint consists of a smooth, glossy substance, called cartilage, covering the two bone surfaces where they meet, providing a low friction and lubricated articulation for ease of motion. The hip flexes, extends, rotates and moves the leg from side to side.
Over a period of years, these smooth surfaces can wear out. The cartilage thins out and breaks down, and unprotected bone surfaces rub against each other. This causes the swelling, inflammation and pain associated with arthritis. Total hip replacement (THR), otherwise known as total hip arthroplasty (THA) surgery, creates a new artificial joint that ultimately can be pain free (Figure 2). The implant is designed to replicate the human anatomy and allow typical motions of the hip. The surgeon inserts a metal cup into the diseased pelvic socket. The smooth inner diameter of this cup, which precisely mates with a large spherical femoral head, is intended to serve the same purpose as natural cartilage, allowing for smooth movement. The diseased ball part of the thigh bone is removed and replaced by, in this case, a relatively large diameter CoCr metal ball mounted onto a neck of the femoral stem, which is fitted snugly into the thigh bone. Finally, the ball is placed securely into the pelvic socket, where it should move easily without causing significant friction or pain. The implant integrates with the natural bone structure and should function like a normal hip.
There are different prosthesis designs for the pelvic socket. One design is a monoblock (one piece), thin-walled, press-fit metallic cup that allows for maximizing head size (to reduce potential for dislocation and reduce bone removal). The Durom Cup is in this category.
This design typically does not utilize supplemental fixation such as screws and spikes. Press-fit fixation produces a tight fit that should minimize gaps at the dome of the cup providing direct contact with the host-bone in the pelvis to allow for bone in-growth. However, potential consequences of thin-walled metallic press-fit cups include cup deformation secondary to insertion forces particularly in younger patients with strong bone. This deformation can lead to changes in bearing geometries resulting in negative tribological consequences.
Additionally, the deformation of the cup at the rim combined with the lack of screw holes provides false seating information
. The cup may seem solidly fixed due to cup deformation particularly at the rim. However, gaps around the cup exist and direct contact against the host bone is absent, leading to poor to no bone in-growth. Since the primary fixation for these types of cups rely on boney in-growth for component stability, the lack of in-growth can result in cup motion causing pain and increased implant wear ultimately leading to early device failure.
The primary total hip replacement (THR) typically lasts 15 years with over a 90% survival rate. The American Association of Hip and Knee Surgeons (AAHKS) report that a primary THR has an 80-85% survival rate at 20 years. This surgery is typically successful at restoring health and well-being to patients. Patients of successful primary THR surgery report physical and psychosocial improvements four to five years post implantation with better social function. Patients currently receiving total joint replacements are younger and more active than in the past. With life expectancies increasing, revision total hip replacement surgeries (to replace the previous implant) are anticipated.
Revision surgeries have less favorable functional outcome and patient satisfaction compared with primary THR. Revision THR have more short and long term complications, higher mortality rates, smaller improvements in functional outcome and lower patient satisfaction. The prosthesis longevity decreases from a 90% survival rate at 15 years with the primary implant to as low as 35% survival rate at 10 years with a revised implant and shorter life duration with every subsequent revision. The risk of re-revision (subsequent revisions) following revision THR exceeds that of revision following primary THR. The failure of a primary THR and subsequent revisions threaten patients with significant morbidity, mortality and reduced quality of life. Since revision surgeries are more complicated, risky and costly than the primary surgery, there is significant emphasis to extend the longevity of the initial implant in order to reduce the number of revisions a patient may require.
The Zimmer Durum Cup
is a large diameter metal-on-metal (MOM) articulation between the cup and head. Design features of the cup include:
- Monoblock, non-modular (one piece)
- Wrought-forged high carbon cobalt-chromium alloy
- Titanium plasma spray coating with roughness of 20-50um on acetabular side for bone in-growth
- 3.7mm thick walls with 0.3mm of titanium plasma sprayed coating
- 165 degrees of head coverage
- 150um clearance (70um radial clearance)
- Flattened dome that flares toward the periphery so it is 2mm wider at the rim than the dome
- 2mm wide sharp circumferential fins that extend 1mm from the edge of the component for rotational stability
- Equatorial peripheral fins are impacted into bone
- No screw holes
- Press-fit with impaction
The Durom cup was approved by the FDA in April 2006. On 22 July 2008, Zimmer released an Urgent Device Correction
letter to orthopedic surgeons stating:
- The results of our in-depth investigation have led us to conclude that additional surgical technique instructions and training are necessary in the United States, and we strongly recommend that U.S. surgeons stop implanting the Durom Cup until receiving such training.
- Zimmer will suspend marketing and distribution of the Durom Cup in the U.S., while we update product labeling to provide more detailed surgical technique instructions and implement a surgical training program for U.S. surgeons.
- The Durom Cup will continue to be marketed and distributed without interruption outside the U.S.
Due to the high incidence of early failure and unacceptably high revision rates, the Durom Cup was voluntarily withdrawn from the market5. Since at least 2007, surgeons implanting the Durom Cup complained to Zimmer that the device was failing in their patients, many of whom had to undergo painful, invasive and expensive revision surgeries.
One of these surgeons was Dr. Lawrence Dorr, a world-renowned orthopedic surgeon and a Zimmer consultant, who warned Zimmer in 2007 of the high rate of Durom Cup failures. At the time Dr. Dorr warned Zimmer of the high rate of failures, he was a veteran of thousands of hip replacement surgeries.
In particular, Dr. Dorr informed Zimmer that x-rays showed that the Durom Cup was failing because it was separating or loosening from the bone, rather than fusing to it, causing patients crippling pain while the metal cup moved around in the hip socket and rubbed against the bone.
Zimmer ignored Dr. Dorr's warnings and continued to sell the Durom Cup.
As a result of Zimmer ignoring Dr. Dorr's warnings, on April 22, 2008, Dr. Dorr publicly warned other orthopedic surgeons about the cup failures his patients were experiencing and urged Zimmer to stop selling the Durom Cup. See Dr. Dorr's Warning
The Durom Cup was voluntarily recalled from the market by Zimmer on September 26, 2008. Zimmer listed the reason for recall as "Instructions for use/surgical technique instructions are inadequate". FDA labeled the determined cause as "Misbranding: Labeling False and Misleading"
Further, Dr. Dorr, and a team of doctors at The Arthritis Institute at Good Samaritan Hospital in Los Angeles, published the results of their study comparing one hundred and eighty-one patients who had the large-diameter (44- to 50-mm) Durom Cup and fifty-four patients who had a small-diameter (28-mm Metasul®) articulation implanted between May 2006 and November 2007. The total number of clinical failures was forty-one of one hundred and eighty-one patients (23%). Twenty-eight of one hundred and fifty-one patients had radiographic impending failure at final follow-up (18.5%). All post-revision surgery retrieved cups were examined in detail and had no evidence of bone on the fixation surface. See Dorr Study
The 23% failure rate found by Dr. Dorr and his associates was four (4) times the rate of failure which was predicted by Zimmer.
When introducing the Durom Cup, Zimmer represented that the Durom Cup would provide greater range of motion and less wear on the bearing than traditional hip replacement implant components, thus making it an ideal product for younger, active patients.
Contrary to Zimmer's representations, the Durom Cup is prone to an unprecedented failure rate for hip replacement implant components.
Since Zimmer first began selling the Durom Cup in the United States from 2006 through approximately July 22, 2008, the product labeling and product information for the Durom Cup failed to contain adequate information, instructions, and warnings concerning implantation of the product and the risks that the Durom Cup can loosen and separate from the acetabulum (hip socket) in patients.
Despite their knowledge of the serious injuries associated with use of the Durom Cup, Zimmer engaged in a marketing and advertising program which, as a whole, by affirmative and material misrepresentations and omissions, falsely and deceptively sought to create the image and impression that the use of the Durom Cup was safe.
Helping Defective Hip Replacement Recipients In Wisconsin