Evolution of Cervical Discs
– A Clinical Overview
Invited lecture given at the 4th International Symposium for Cervical and Lumbar Arthroplasty
Berlin, October 7th 2016
Berlin, October 7th 2016
Abstract
The evolution of cervical disc arthroplasty has been a complex, iterative process, driven by passionate clinicians and supported by innovative engineers. As with so many historical advances, disc arthroplasty has suffered failures and controversy.
The first cervical disc prostheses were probably the 13 stainless steel balls that Ulf Fernström implanted into 8 Swedish patients in the early 1960’s. Critical of his own results, citing instability and subsidence, and facing fierce criticism from colleagues such as Alf Nachemson who apparently favoured an elastomeric device, Fernström abandoned the procedure.
A controversial figure in the early history of cervical disc arthroplasty was the Canadian surgeon, Dr J David Kuntz. Granted a US patent in 1982 for a 2-piece disc prosthesis that bears strong resemblance to subsequent commercially available devices, he apparently made his own single-piece cervical disc prostheses in the 1970’s and reported having implanted some 521 of these into 300 patients in the early 1980’s.
While numerous prosthetic disc designs were patented over the succeeding two decades, the next clinical chapter took place at the Frenchay Hospital in Bristol, UK where a 2-piece, stainless steel, (superior)ball-in-(inferior)socket device with anteriorly placed anchoring screws was developed and first implanted in 1991. Integral to the clinical success of any arthroplasty device are its biomechanics. In hindsight, the biomechanics of the Bristol device were questionable and several patients suffered dislodgement and screw breakages. The device was revised, acquired and further modified by Medtronic to a ball-in-trough configuration, becoming the Prestige disc. This device was the first cervical disc prosthesis to be granted FDA approval in the U.S. With an interesting 5 degree-of-freedom kinematic profile and now made of Titanium, it continues to enjoy a loyal following.
A substantial evolutionary step occurred at the turn of the century when the Seattle neurosurgeon, Dr Vince Bryan developed a device incorporating a sliding, bi-convex, polyurethane core and titanium endplate shells, similar in kinematic concept to the lumbar Charité device. Dr Bryan designed a complex gravity-referenced stereotactic milling system to enable precise surgical implantation of the prosthesis. Like the Prestige disc, with multiple degrees-of-freedom, the Bryan device proved attractive in providing a joint with a variable instantaneous center of rotation. The disc continues to have a loyal following but many disliked the complex milling system and there were reports of post-implantation segmental kyphosis. The reasons for the device kyphosis are multifactorial but predominantly and likely the result of excessive bone milling during implantation, in some patients, rather than an inherent fault in the device.
A number of (inferior)ball-in-(superior)socket devices have been developed and marketed, beginning with the Prodisc-C, which was also granted FDA approval in 2007 following an RCT demonstrating equivalence in 2-year clinical outcomes to the ‘gold’ standard of ACDF. These 3 degree-of-freedom devices with fixed center of rotation (COR) have proven clinically successful, at least in the short to medium term, so long as the device COR approximates the patient’s inherent, segmental COR. Devices where this is not the case have been prone to migration, loosening or expulsion and have become unpopular.
Ongoing research and development into implant materials and biomechanics are resulting in newer generation devices that more closely mimic the natural segmental quantity and quality of motion. It is vital that in the quest to avoid adjacent segment disease that other problems aren’t created, such as same segment disease. Surgeons are looking for devices that are easy to implant, restore spinal alignment, match as closely as possible the inherent segmental biomechanics, facilitate subsequent spinal imaging and above all, are safe. The optimal implant and instrument designs will not have yet been reached. Clinical decision processes continue to be researched – when to implant prosthetic discs; whether to implant at one or multiple levels; whether to carry out hybrid procedures.
Recent figures indicate that approximately 15,000 cervical disc replacements are now done annually in the U.S. In Australia, where outspoken opponents of cervical disc arthroplasty used the regulatory process to close it down between 2006 and 2011, the procedural numbers have been almost doubling every year since and the National Joint Replacement Registry provides some fascinating insights into the clinical evolution.
While critics are now less vocal and despite all the progress to date, confirmation remains elusive of cervical disc arthroplasty’s ability to achieve its ultimate goal – reduction in the incidence of adjacent segment disease. The story continues to evolve.
The evolution of cervical disc arthroplasty has been a complex, iterative process, driven by passionate clinicians and supported by innovative engineers. As with so many historical advances, disc arthroplasty has suffered failures and controversy.
The first cervical disc prostheses were probably the 13 stainless steel balls that Ulf Fernström implanted into 8 Swedish patients in the early 1960’s. Critical of his own results, citing instability and subsidence, and facing fierce criticism from colleagues such as Alf Nachemson who apparently favoured an elastomeric device, Fernström abandoned the procedure.
A controversial figure in the early history of cervical disc arthroplasty was the Canadian surgeon, Dr J David Kuntz. Granted a US patent in 1982 for a 2-piece disc prosthesis that bears strong resemblance to subsequent commercially available devices, he apparently made his own single-piece cervical disc prostheses in the 1970’s and reported having implanted some 521 of these into 300 patients in the early 1980’s.
While numerous prosthetic disc designs were patented over the succeeding two decades, the next clinical chapter took place at the Frenchay Hospital in Bristol, UK where a 2-piece, stainless steel, (superior)ball-in-(inferior)socket device with anteriorly placed anchoring screws was developed and first implanted in 1991. Integral to the clinical success of any arthroplasty device are its biomechanics. In hindsight, the biomechanics of the Bristol device were questionable and several patients suffered dislodgement and screw breakages. The device was revised, acquired and further modified by Medtronic to a ball-in-trough configuration, becoming the Prestige disc. This device was the first cervical disc prosthesis to be granted FDA approval in the U.S. With an interesting 5 degree-of-freedom kinematic profile and now made of Titanium, it continues to enjoy a loyal following.
A substantial evolutionary step occurred at the turn of the century when the Seattle neurosurgeon, Dr Vince Bryan developed a device incorporating a sliding, bi-convex, polyurethane core and titanium endplate shells, similar in kinematic concept to the lumbar Charité device. Dr Bryan designed a complex gravity-referenced stereotactic milling system to enable precise surgical implantation of the prosthesis. Like the Prestige disc, with multiple degrees-of-freedom, the Bryan device proved attractive in providing a joint with a variable instantaneous center of rotation. The disc continues to have a loyal following but many disliked the complex milling system and there were reports of post-implantation segmental kyphosis. The reasons for the device kyphosis are multifactorial but predominantly and likely the result of excessive bone milling during implantation, in some patients, rather than an inherent fault in the device.
A number of (inferior)ball-in-(superior)socket devices have been developed and marketed, beginning with the Prodisc-C, which was also granted FDA approval in 2007 following an RCT demonstrating equivalence in 2-year clinical outcomes to the ‘gold’ standard of ACDF. These 3 degree-of-freedom devices with fixed center of rotation (COR) have proven clinically successful, at least in the short to medium term, so long as the device COR approximates the patient’s inherent, segmental COR. Devices where this is not the case have been prone to migration, loosening or expulsion and have become unpopular.
Ongoing research and development into implant materials and biomechanics are resulting in newer generation devices that more closely mimic the natural segmental quantity and quality of motion. It is vital that in the quest to avoid adjacent segment disease that other problems aren’t created, such as same segment disease. Surgeons are looking for devices that are easy to implant, restore spinal alignment, match as closely as possible the inherent segmental biomechanics, facilitate subsequent spinal imaging and above all, are safe. The optimal implant and instrument designs will not have yet been reached. Clinical decision processes continue to be researched – when to implant prosthetic discs; whether to implant at one or multiple levels; whether to carry out hybrid procedures.
Recent figures indicate that approximately 15,000 cervical disc replacements are now done annually in the U.S. In Australia, where outspoken opponents of cervical disc arthroplasty used the regulatory process to close it down between 2006 and 2011, the procedural numbers have been almost doubling every year since and the National Joint Replacement Registry provides some fascinating insights into the clinical evolution.
While critics are now less vocal and despite all the progress to date, confirmation remains elusive of cervical disc arthroplasty’s ability to achieve its ultimate goal – reduction in the incidence of adjacent segment disease. The story continues to evolve.
