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Modularity in primary total hip arthroplasty
The Journal of Arthroplasty Vol. 11 No. 3 1996
Point-Counterpoint
Modularity in Primary Total Hip Arthroplasty
Cemented stems are not required to accurately reproduce the geometry of the femoral canal as the gap is filled with cement. The introduction of noncemented, primarily porous-coated, hips changed that requirement. Animal experiments and clinical results have shown that the closer the porous metal coating came to the endosteal cortical bone, the more rapid and complete bone ingrowth was. Good cortical contact over a sufficient length was also required for initial stability. The concept of fit and fill thus arose, the objective being to fit the implant to the patient such that it filled the canal. A certain adjustment of canal size is possible by reaming, but this is limited to the thickness of the cortical bone. Monolithic stems can be fitted quite successfully to a fair percentage of younger people with type A bone, but difficulties are experienced in poor-quality bone in revision surgery. There is no consistent relationship between the diameters of the diaphysis and metaphysis. In a vain attempt to produce a monolithic stem that would fit a fair percentage of canals, sales figures for the size and shape of the S-ROM sleeves (Joint Medical Products, Stamford, CT) for 1 year were plotted against stem diameter. It was hoped that a bell curve could be produced for each stem diameter. The figures, however, were all over the map, indicating that there is no optimum geometry that fits the majority of canals. To even remotely fill the canal therefore, a vast n u m b e r of implants would have to be available with significantly different metaphyseal and diaphyseal sizes and shapes. One way around this problem is to ignore the metaphysis completely and ensure diaphyseal fill over a sufficient distance with a fully porous-coated implant. Although undoubtedly fixation can be achieved by this method, it does introduce the problem of proximal stress shielding above the most distal point of fixation and considerable difficulties in implant removal should revision be required. Proximal fixation by bone ingrowth is theoretically more attractive as it limits the length of stressshielded segment of bone and makes removal much easier with an extended trochanteric osteotomy or slide. 4 To achieve proximal fixation reliably, however, the stem must give distal stability. The only reasonable way of doing this is a modular system. In a proximal modular system, which is the only system with which I have experience, the stem fills the dia-
Modularity in Primary Total Hip Arthroplasty Hugh U. Cameron, MB, ChB, FRCS (C) History.
Sixteen years ago, modular head/neck junctions were introduced into North America. 1 The advantages were the ability to combine different materials for the head and stem (in this case ceramic and metal); the ability to allow equalization of leg lengths after the final stem had been inserted; and the ability to reduce inventory and costs. It subsequently became obvious that modular heads had further advantages. In isolated acetabular revisions, the head could be removed to allow visualization and then replaced. If instability was present, a longer neck or a larger head could be substituted. Where a high-hipcenter acetabular placement was the position of choice, stem extensions could be applied, 2 leaving a well-fixed femoral component. In general, minimal disadvantages have been found with head/stem modularity. Some corrosion has been found in smaller tapers where the initial taper geometry was not as well controlled as it might have been. Some fretting or micromotion of one component on another with consequent release of metallic particles is inevitable. Other than seven reported cases of head/neck dissociation during attempted closed reduction of a dislocation and a few reported cases of a l u m i n u m ceramic ball fracture, 3 no clinical problems have been noted as a result of the introduction of modular heads. Skirted components should be avoided if possible as they reduce head/neck diameter difference, thus decreasing the range of movement. It is, therefore, preferable that a hip system have variation in neck length built into the stem. Few surgeons would trade the ease and convenience of modular heads to return to fixed head components.
From the Departments of Surgery, Pathology, and Engineering, Universify of Toronto, and Orthopaedic & Arthritic Hospital, Toronto, Ontario, Canada. Reprint requests: Hugh U. Cameron, 43 Wellesley Street, East Suite 318, Toronto, Ontario M4Y 1H1, Canada.
332
Point-Counterpoint: Modularity in PrimaryTHA physis and is undersized proximally. Canal fill is then achieved by having a series of geometrically different sleeves that can be attached to the stem. One potential advantage of proximal modularity is version change. Because the stem is undersized proximally, it can be inserted with any degree of version with respect to the metaphysis. The sleeve can then be coupled at any angle to allow full metaphyseal fill. This ability to change version is of particular importance in cases where rotational malalignment of the proximal femur exists as, for example, in congenital dysplasia of the hip (CDH) or following injury or osteotomy. The indication for use of a modular n o n c e m e n t e d system is simply a case in which the use of cement is undesirable for whatever reason and in which templating has shown that a monolithic stem fits poorly. The only problem is that although distal templating is easy and accurate, metaphyseal templating is exceedingly inaccurate. It is necessary to template the metaphysis in two planes. Templating in the lateral radiograph is significantly affected by the degree of bowing of the femur. Faced with this difficulty, the author uses a proximally modular stem in all n o n c e m e n t e d cases. Many of these indications refer only to the S-ROM stem, which has both proximal and distal rotational stability. 5 An indication is version change, which may be required in a CDH or revision case. Osteotomies of the femur require proximal and distal angular and rotational stability.4,6,7 Concerns obviously exist about coupling dissociation. Most couplings are Morse tapers and the tapers must be locked by firm h a m m e r blows. Care must be taken to avoid the presence of particulate debris in the couplings such as bone-cement, which may prevent taper locking. I have never experienced this problem. Coupling fracture remains a theoretical possibility, but no coupling fracture has been reported to date. All nonwelded metal-metal junctions must have some degree of micromotion under load. This results in the liberation of metallic debris. Wet testing in the laboratory has suggested that about 2 million particles per year are liberated from the S-ROM junction. This pales into insignificance w h e n compared with the 2 billion particles of polyethylene liberated per year from an apparently normal acetabular component. Analysis of retrieved SROM specimens has shown less fretting than was predicted from laboratory studies. A comparison by Bobyn of S-ROM head/neck junctions with sleeve/stem junctions showed little difference, s The only sleeve that demonstrated significant wear was the one in which b o n e cement had prevented taper locking.
Results. The results refer to primary cases I performed using ingrowth components in the femur and acetabulum. Revisions of a previous total hip arthroplasty have been excluded. There were 208 cases; 4 were lost to follow-up evaluation at less than 2 years and 2 cases had the stems revised, 1 for collapse of a napkin ring occasioned by a subtrochanteric osteotomy and 1 for fracture below the stem tip, leaving 202 cases with a follow-up period of 2 to 8 years. There were 52 m e n and 156
333
women. The age range was 17 to 67 years (mean, 41 years). The original diagnoses were primary osteoarthritis in 78, CDH in 74, Perthes' disease in 9, old septic in 10, and a range of other less common conditions. Prior surgeries included l l fusions, 8 pelvic osteotomies, 6 intertrochanteric osteotomies, 2 shelves, 4 subtrochanteric osteotomies, 2 pin-and-plate procedures, and 1 femoral lengthening. Concomitant operations at the time of total hip arthroplasty included 38 structural acetabular grafts, 8 nonstructural grafts, 4 subtrochanteric osteotomies, 1 diaphyseal osteotomy, 2 trochanteric osteotomies, 2 calcar episiotomies, and 2 stem shortenings. Intraoperative complications included 12 crack fractures of the calcar, 7 of which extended below the lesser trochanter and were therefore wired. There were two crack fractures of the greater trochanter, two femoral perforations, four femoral nerve injuries, and one sciatic nerve injury. Postoperative complications included one delayed w o u n d healing, one collapse of a napkin ring, one fracture below the stem tip, two dislocations treated by closed reduction, one fracture through an area of osteolysis, and one myocardial infarct. The Harris ratings were 88.6% excellent, 6.9% good, 3% fair, and 1.5% poor. The fair and poor cases included one cerebrovascular accident 2 years after surgery, two previously fused hips, five high CDH cases, and two cases of nerve dysesthesia. Limp was absent in 85.2% of cases, slight in 8.4%, and severe (ie, positive Trendelenburg test) in 6.4%. Pain was absent in 91.6%, insignificant (ie, required no analgesics) in 6.4%, and moderate in 2%. The pain site was the greater trochanter in 2.5%, groin/buttock in 3.5%, and femur in 2%. Lucency was type IA (ie, no lucency) in 87.6% and type IB (ie, incomplete lucency) in 12.4%. There were no type II or III cases. Of the 25 (12.4%) cases with partial lucency, a single zone was involved in 19, two zones in 3, and three zones in 3. Osteolysis was present above the sleeve (ie, the fixation point) in 15 (7.4%) cases, with 10 being in the greater trochanter and 5 in the calcar. No area of osteolysis exceeded 6 m m in diameter. One case was found to have osteolysis below the stem! sleeve junction. This case developed lucency very early in the ingrowth zone, presumably allowing passage of polyethylene distal to the sleeve. She subsequently had a fracture through the area of osteolysis. She was treated conservatively and went on to heal. Distal stability of the stem has been preserved, but in light of this area of osteolysis, revision is planned.
Discussion. Osteolysis above the level of fixation is almost inevitable as long as polyethylene is in the equation. As only one case in this series has shown distal osteolysis, it can be concluded that at least in the time frame studied of up to 8 years, bone ingrowth around the sleeve serves as an adequate gasket and the particulate debris does not leak through the taper junction, at least in quantities sufficient to produce any clinical effect. Proximal modularity has rendered very easy some of these technically very difficult cases. As with almost a decade of experience with this system, the incidence of
334
The Journal of Arthroplasty Vol. 11 No. 3 April 1996
implant-related complications is very small, I would be exceedingly unwilling to give up the proven advantages of modularity for the theoretical disadvantages.
References 1. Cameron HU, Loehr J, Fornasier VL: Early clinical trials with a ceramic total hip prosthesis. Orthop Rev 12:49, 1983 2. Cameron HU: Use of a neck extension sleeve in total hip replacement. Contemp Orthop 21:67, 1990 3. Cameron HU: Ceramic head fractures in total hip replacement. J Arthroplasty 6:185, 1991 4. Cameron HU: Use of a distal trochanteric osteotomy in hip revisions. Contemp Orthop 23:235, 1991 5. Cameron HU, Bhimji S: Early clinical trials with a proximally fixed uncemented hip stem. Contemp Orthop 17:31, 1988 6. Cameron HU: Proximal femoral osteotomy in difficult revision hip surgery: how to revise the unrevisable. Contemp Orthop 28:565, 1989 7. Cameron HU: Use of a distally fluted long stem prosthesis in the correction of angular deformity of the femur. Contemp Orthop 20:159, 1990 8. Bobyn JD, Dujovne AR, Figgie JJ, Young DC: Surface analysis ot the taper junctions of retrieved and in vitro tested modular hip prostheses systems, p. 287. In Morrey BE (ed): Biology, materials, and mechanical considerations of joint replacement. Lippincott-Raven, NY, 1993
Modularity Is Unnecessary in Primary Femoral THA But Has Some Advantages in Primary AcetabularTHA William H. Harris, MD At first blush, modularity appears to be such an overwhelming advantage that m a n y people would question the subject of this Point-Counterpoint: Modularity in Primary Total Hip Arthroplasty. Some would feel "why even ask the question?" This attitude represents an example of the dominating power of the status quo and, perhaps, is a reflection of the absence of critical thinking on this issue. Rigorous consideration of the value and disadvantages of modularity should be separated into two headings, femoral modularity and acetabular modularity. The factors involved in each are quite disparate.
Femoral Modularity. The advantages and disadvantages of femoral modularity in primary total hip arthroplasty (THA) need to be assessed in terms of both the primary operation itself and any subsequent revision that is required. In relation to this, it is important to realize two facts. First, nearly 85% of THA operations are primary. Second, among revision operations, in about 85% of cases, the femoral component is changed. These two From the Orthopaedic Biomechanics Laboratory and the Hip and Implant Unit of the Department of Orthopaedic Surgery of Massachusetts General Hospital, Boston, Massachusetts. Reprint requests: William H. Harris, MD, Orthopaedic Biomechanics Laboratory, GrJ 1126, Massachusetts General Hospital, 32 Fruk Street, Boston, MA 02114.
facts are crucial, as I will show that femoral modularity is not advantageous in primary THA, and that in most revisions, because the femoral component is changed, femoral modularity has no value in these cases either. Consider, initially, the primary operation itsell. Although modularity of the femoral component is nearly ubiquitous, thoughtful assessment rapidly focuses on the fact that modularity has major disadvantages in virtually every other aspect except inventory. Many surgeons reply that the modularity is essential for the flexibility necessary to select the optimum neck length and head size. Further reflection, however, forces the realization that w h e n one considers the stages of the operative process, this is simply u n t r u e in most cases. Flexibility is, indeed, important. But in reality it is necessary only during the trial portion of the operation, to determine appropriate neck length, head size, range of motion, stability, and abductor tension. None of that flexibility, however, is required in the implant itself. The needed flexibility is provided by the provisional components used in the trial reduction. No flexibility is needed beyond that. Another way of saying the same thing is to point out that there is, ultimately, no difference between deciding (based on your trial reduction) on using a 2 8 - m m head of m e d i u m length and assembling a modular 28-mm head of m e d i u m length on the Morse taper of a modular stem on the back table or alternatively, simply calling for a femoral component of monoblock construction with a 28-ram head and medium length. In either case, the modularity needed to make the decisions is provided by the modular provisional pieces. Modularity in the implant is not needed for this purpose. If, beyond that, there are no advantages, then what are the disadvantages? They are m a n y and impressive. In the first instance, the monoblock neck can be made much thinner than a Morse taper. That results in a sequence of advantages: range of motion is better, dislocation is reduced, impingement of the neck against the polyethylene is reduced, wear from that impingement is reduced, stress transmitted from such impingement to the bone-prosthesis interface behind the acetabulum is reduced, and stress against the interface between the polyethylene and the shell in a modular acetabular comp o n e n t is reduced. There is a substantial array of advantages. Dislocation is among the most common postoperative complications of primary THA. The risk of dislocation is markedly reduced by having a monoblock neck construction. Compared with similar implants of modular construction, the range of motion in a monoblock stem averages about 15 ° greater for a short or medium neck length and about 35 ° for a long neck length. To some surgeons, this i5 ° or 35 ° increase in range may seem small but, in fact, these degrees are the most valuable in the entire arc. They are the degrees that prevent impingement and thus reduce the risk of dislocation. Impingement is related to the ratio of the head diameter to the neck diameter. With the extensive shift in usage away from the 32-mm femoral heads, the advan-
Point-Counterpoint
Modularity in Primary Total Hip Arthroplasty
Cemented stems are not required to accurately reproduce the geometry of the femoral canal as the gap is filled with cement. The introduction of noncemented, primarily porous-coated, hips changed that requirement. Animal experiments and clinical results have shown that the closer the porous metal coating came to the endosteal cortical bone, the more rapid and complete bone ingrowth was. Good cortical contact over a sufficient length was also required for initial stability. The concept of fit and fill thus arose, the objective being to fit the implant to the patient such that it filled the canal. A certain adjustment of canal size is possible by reaming, but this is limited to the thickness of the cortical bone. Monolithic stems can be fitted quite successfully to a fair percentage of younger people with type A bone, but difficulties are experienced in poor-quality bone in revision surgery. There is no consistent relationship between the diameters of the diaphysis and metaphysis. In a vain attempt to produce a monolithic stem that would fit a fair percentage of canals, sales figures for the size and shape of the S-ROM sleeves (Joint Medical Products, Stamford, CT) for 1 year were plotted against stem diameter. It was hoped that a bell curve could be produced for each stem diameter. The figures, however, were all over the map, indicating that there is no optimum geometry that fits the majority of canals. To even remotely fill the canal therefore, a vast n u m b e r of implants would have to be available with significantly different metaphyseal and diaphyseal sizes and shapes. One way around this problem is to ignore the metaphysis completely and ensure diaphyseal fill over a sufficient distance with a fully porous-coated implant. Although undoubtedly fixation can be achieved by this method, it does introduce the problem of proximal stress shielding above the most distal point of fixation and considerable difficulties in implant removal should revision be required. Proximal fixation by bone ingrowth is theoretically more attractive as it limits the length of stressshielded segment of bone and makes removal much easier with an extended trochanteric osteotomy or slide. 4 To achieve proximal fixation reliably, however, the stem must give distal stability. The only reasonable way of doing this is a modular system. In a proximal modular system, which is the only system with which I have experience, the stem fills the dia-
Modularity in Primary Total Hip Arthroplasty Hugh U. Cameron, MB, ChB, FRCS (C) History.
Sixteen years ago, modular head/neck junctions were introduced into North America. 1 The advantages were the ability to combine different materials for the head and stem (in this case ceramic and metal); the ability to allow equalization of leg lengths after the final stem had been inserted; and the ability to reduce inventory and costs. It subsequently became obvious that modular heads had further advantages. In isolated acetabular revisions, the head could be removed to allow visualization and then replaced. If instability was present, a longer neck or a larger head could be substituted. Where a high-hipcenter acetabular placement was the position of choice, stem extensions could be applied, 2 leaving a well-fixed femoral component. In general, minimal disadvantages have been found with head/stem modularity. Some corrosion has been found in smaller tapers where the initial taper geometry was not as well controlled as it might have been. Some fretting or micromotion of one component on another with consequent release of metallic particles is inevitable. Other than seven reported cases of head/neck dissociation during attempted closed reduction of a dislocation and a few reported cases of a l u m i n u m ceramic ball fracture, 3 no clinical problems have been noted as a result of the introduction of modular heads. Skirted components should be avoided if possible as they reduce head/neck diameter difference, thus decreasing the range of movement. It is, therefore, preferable that a hip system have variation in neck length built into the stem. Few surgeons would trade the ease and convenience of modular heads to return to fixed head components.
From the Departments of Surgery, Pathology, and Engineering, Universify of Toronto, and Orthopaedic & Arthritic Hospital, Toronto, Ontario, Canada. Reprint requests: Hugh U. Cameron, 43 Wellesley Street, East Suite 318, Toronto, Ontario M4Y 1H1, Canada.
332
Point-Counterpoint: Modularity in PrimaryTHA physis and is undersized proximally. Canal fill is then achieved by having a series of geometrically different sleeves that can be attached to the stem. One potential advantage of proximal modularity is version change. Because the stem is undersized proximally, it can be inserted with any degree of version with respect to the metaphysis. The sleeve can then be coupled at any angle to allow full metaphyseal fill. This ability to change version is of particular importance in cases where rotational malalignment of the proximal femur exists as, for example, in congenital dysplasia of the hip (CDH) or following injury or osteotomy. The indication for use of a modular n o n c e m e n t e d system is simply a case in which the use of cement is undesirable for whatever reason and in which templating has shown that a monolithic stem fits poorly. The only problem is that although distal templating is easy and accurate, metaphyseal templating is exceedingly inaccurate. It is necessary to template the metaphysis in two planes. Templating in the lateral radiograph is significantly affected by the degree of bowing of the femur. Faced with this difficulty, the author uses a proximally modular stem in all n o n c e m e n t e d cases. Many of these indications refer only to the S-ROM stem, which has both proximal and distal rotational stability. 5 An indication is version change, which may be required in a CDH or revision case. Osteotomies of the femur require proximal and distal angular and rotational stability.4,6,7 Concerns obviously exist about coupling dissociation. Most couplings are Morse tapers and the tapers must be locked by firm h a m m e r blows. Care must be taken to avoid the presence of particulate debris in the couplings such as bone-cement, which may prevent taper locking. I have never experienced this problem. Coupling fracture remains a theoretical possibility, but no coupling fracture has been reported to date. All nonwelded metal-metal junctions must have some degree of micromotion under load. This results in the liberation of metallic debris. Wet testing in the laboratory has suggested that about 2 million particles per year are liberated from the S-ROM junction. This pales into insignificance w h e n compared with the 2 billion particles of polyethylene liberated per year from an apparently normal acetabular component. Analysis of retrieved SROM specimens has shown less fretting than was predicted from laboratory studies. A comparison by Bobyn of S-ROM head/neck junctions with sleeve/stem junctions showed little difference, s The only sleeve that demonstrated significant wear was the one in which b o n e cement had prevented taper locking.
Results. The results refer to primary cases I performed using ingrowth components in the femur and acetabulum. Revisions of a previous total hip arthroplasty have been excluded. There were 208 cases; 4 were lost to follow-up evaluation at less than 2 years and 2 cases had the stems revised, 1 for collapse of a napkin ring occasioned by a subtrochanteric osteotomy and 1 for fracture below the stem tip, leaving 202 cases with a follow-up period of 2 to 8 years. There were 52 m e n and 156
333
women. The age range was 17 to 67 years (mean, 41 years). The original diagnoses were primary osteoarthritis in 78, CDH in 74, Perthes' disease in 9, old septic in 10, and a range of other less common conditions. Prior surgeries included l l fusions, 8 pelvic osteotomies, 6 intertrochanteric osteotomies, 2 shelves, 4 subtrochanteric osteotomies, 2 pin-and-plate procedures, and 1 femoral lengthening. Concomitant operations at the time of total hip arthroplasty included 38 structural acetabular grafts, 8 nonstructural grafts, 4 subtrochanteric osteotomies, 1 diaphyseal osteotomy, 2 trochanteric osteotomies, 2 calcar episiotomies, and 2 stem shortenings. Intraoperative complications included 12 crack fractures of the calcar, 7 of which extended below the lesser trochanter and were therefore wired. There were two crack fractures of the greater trochanter, two femoral perforations, four femoral nerve injuries, and one sciatic nerve injury. Postoperative complications included one delayed w o u n d healing, one collapse of a napkin ring, one fracture below the stem tip, two dislocations treated by closed reduction, one fracture through an area of osteolysis, and one myocardial infarct. The Harris ratings were 88.6% excellent, 6.9% good, 3% fair, and 1.5% poor. The fair and poor cases included one cerebrovascular accident 2 years after surgery, two previously fused hips, five high CDH cases, and two cases of nerve dysesthesia. Limp was absent in 85.2% of cases, slight in 8.4%, and severe (ie, positive Trendelenburg test) in 6.4%. Pain was absent in 91.6%, insignificant (ie, required no analgesics) in 6.4%, and moderate in 2%. The pain site was the greater trochanter in 2.5%, groin/buttock in 3.5%, and femur in 2%. Lucency was type IA (ie, no lucency) in 87.6% and type IB (ie, incomplete lucency) in 12.4%. There were no type II or III cases. Of the 25 (12.4%) cases with partial lucency, a single zone was involved in 19, two zones in 3, and three zones in 3. Osteolysis was present above the sleeve (ie, the fixation point) in 15 (7.4%) cases, with 10 being in the greater trochanter and 5 in the calcar. No area of osteolysis exceeded 6 m m in diameter. One case was found to have osteolysis below the stem! sleeve junction. This case developed lucency very early in the ingrowth zone, presumably allowing passage of polyethylene distal to the sleeve. She subsequently had a fracture through the area of osteolysis. She was treated conservatively and went on to heal. Distal stability of the stem has been preserved, but in light of this area of osteolysis, revision is planned.
Discussion. Osteolysis above the level of fixation is almost inevitable as long as polyethylene is in the equation. As only one case in this series has shown distal osteolysis, it can be concluded that at least in the time frame studied of up to 8 years, bone ingrowth around the sleeve serves as an adequate gasket and the particulate debris does not leak through the taper junction, at least in quantities sufficient to produce any clinical effect. Proximal modularity has rendered very easy some of these technically very difficult cases. As with almost a decade of experience with this system, the incidence of
334
The Journal of Arthroplasty Vol. 11 No. 3 April 1996
implant-related complications is very small, I would be exceedingly unwilling to give up the proven advantages of modularity for the theoretical disadvantages.
References 1. Cameron HU, Loehr J, Fornasier VL: Early clinical trials with a ceramic total hip prosthesis. Orthop Rev 12:49, 1983 2. Cameron HU: Use of a neck extension sleeve in total hip replacement. Contemp Orthop 21:67, 1990 3. Cameron HU: Ceramic head fractures in total hip replacement. J Arthroplasty 6:185, 1991 4. Cameron HU: Use of a distal trochanteric osteotomy in hip revisions. Contemp Orthop 23:235, 1991 5. Cameron HU, Bhimji S: Early clinical trials with a proximally fixed uncemented hip stem. Contemp Orthop 17:31, 1988 6. Cameron HU: Proximal femoral osteotomy in difficult revision hip surgery: how to revise the unrevisable. Contemp Orthop 28:565, 1989 7. Cameron HU: Use of a distally fluted long stem prosthesis in the correction of angular deformity of the femur. Contemp Orthop 20:159, 1990 8. Bobyn JD, Dujovne AR, Figgie JJ, Young DC: Surface analysis ot the taper junctions of retrieved and in vitro tested modular hip prostheses systems, p. 287. In Morrey BE (ed): Biology, materials, and mechanical considerations of joint replacement. Lippincott-Raven, NY, 1993
Modularity Is Unnecessary in Primary Femoral THA But Has Some Advantages in Primary AcetabularTHA William H. Harris, MD At first blush, modularity appears to be such an overwhelming advantage that m a n y people would question the subject of this Point-Counterpoint: Modularity in Primary Total Hip Arthroplasty. Some would feel "why even ask the question?" This attitude represents an example of the dominating power of the status quo and, perhaps, is a reflection of the absence of critical thinking on this issue. Rigorous consideration of the value and disadvantages of modularity should be separated into two headings, femoral modularity and acetabular modularity. The factors involved in each are quite disparate.
Femoral Modularity. The advantages and disadvantages of femoral modularity in primary total hip arthroplasty (THA) need to be assessed in terms of both the primary operation itself and any subsequent revision that is required. In relation to this, it is important to realize two facts. First, nearly 85% of THA operations are primary. Second, among revision operations, in about 85% of cases, the femoral component is changed. These two From the Orthopaedic Biomechanics Laboratory and the Hip and Implant Unit of the Department of Orthopaedic Surgery of Massachusetts General Hospital, Boston, Massachusetts. Reprint requests: William H. Harris, MD, Orthopaedic Biomechanics Laboratory, GrJ 1126, Massachusetts General Hospital, 32 Fruk Street, Boston, MA 02114.
facts are crucial, as I will show that femoral modularity is not advantageous in primary THA, and that in most revisions, because the femoral component is changed, femoral modularity has no value in these cases either. Consider, initially, the primary operation itsell. Although modularity of the femoral component is nearly ubiquitous, thoughtful assessment rapidly focuses on the fact that modularity has major disadvantages in virtually every other aspect except inventory. Many surgeons reply that the modularity is essential for the flexibility necessary to select the optimum neck length and head size. Further reflection, however, forces the realization that w h e n one considers the stages of the operative process, this is simply u n t r u e in most cases. Flexibility is, indeed, important. But in reality it is necessary only during the trial portion of the operation, to determine appropriate neck length, head size, range of motion, stability, and abductor tension. None of that flexibility, however, is required in the implant itself. The needed flexibility is provided by the provisional components used in the trial reduction. No flexibility is needed beyond that. Another way of saying the same thing is to point out that there is, ultimately, no difference between deciding (based on your trial reduction) on using a 2 8 - m m head of m e d i u m length and assembling a modular 28-mm head of m e d i u m length on the Morse taper of a modular stem on the back table or alternatively, simply calling for a femoral component of monoblock construction with a 28-ram head and medium length. In either case, the modularity needed to make the decisions is provided by the modular provisional pieces. Modularity in the implant is not needed for this purpose. If, beyond that, there are no advantages, then what are the disadvantages? They are m a n y and impressive. In the first instance, the monoblock neck can be made much thinner than a Morse taper. That results in a sequence of advantages: range of motion is better, dislocation is reduced, impingement of the neck against the polyethylene is reduced, wear from that impingement is reduced, stress transmitted from such impingement to the bone-prosthesis interface behind the acetabulum is reduced, and stress against the interface between the polyethylene and the shell in a modular acetabular comp o n e n t is reduced. There is a substantial array of advantages. Dislocation is among the most common postoperative complications of primary THA. The risk of dislocation is markedly reduced by having a monoblock neck construction. Compared with similar implants of modular construction, the range of motion in a monoblock stem averages about 15 ° greater for a short or medium neck length and about 35 ° for a long neck length. To some surgeons, this i5 ° or 35 ° increase in range may seem small but, in fact, these degrees are the most valuable in the entire arc. They are the degrees that prevent impingement and thus reduce the risk of dislocation. Impingement is related to the ratio of the head diameter to the neck diameter. With the extensive shift in usage away from the 32-mm femoral heads, the advan-