Instability is the most common reason for revision after total hip arthroplasty (THA). Since THA requires arthrotomy of the hip and replacement with a femoral head that is smaller than the normal hip, instability following THA is always a potential concern. Many factors contribute to the development of instability after THA including: restoration of normal anatomy, implant design, component position, surgical approach and technique, and numerous patient related factors. Recently, the role of spinal mobility and deformity has been shown to have a significant effect on risk of dislocation after THA. The long held guidelines for component positioning or so called “safe zone” described by Lewinnek have also been questioned since most dislocations have been shown to occur in patients whose components are positioned within this “safe” range.

In the early post-operative period, dislocation can occur prior to capsular and soft tissue healing if the patient exceeds their peri-operative range of motion limits. Closed reduction and abduction bracing for 6 weeks may allow for soft tissue healing and stabilization of the hip. It is important to try and identify the mechanism of dislocation since this can affect the technique of closed reduction, how the patient is braced following reduction and what may need to be addressed at the time of revision if dislocation recurs. Closed reduction and bracing may be effective in patients who have a previously well-functioning, stable THA who suffer a traumatic dislocation after the peri-operative period. Despite successful closed reduction, recurrent dislocation occurs in many patients and can be secondary to inadequate soft tissue healing, patient noncompliance or problems related to component positioning. Patients who incur more than 2 dislocations should be considered for revision surgery.

Prior to revision surgery, an appropriate radiographic evaluation of the hip should be performed to identify any potential mechanical/kinematic issues that need to be addressed at the time of revision. Typically this involves plain radiographs, including a cross table lateral of the involved hip to assess acetabular version, but may also involve cross-sectional imaging to assess femoral version. Patients with soft tissue pseudotumors frequently have significant soft tissue deficiencies that are not amenable to component repositioning alone and require use of constrained or dual mobility components. In general, “limited revisions” consisting of modular head and liner exchange with insertion of a lipped liner and larger, longer femoral head rarely correct the problem of recurrent instability, since component malposition that frequently contributes to the instability is not addressed. Similarly, insertion of a constrained liner in a malpositioned cup is associated with a high rate of implant failure and recurrent dislocation since impingement contributing to the instability is not addressed.

In patients who fail closed management and have a history of recurrent instability, we have found the treatment paradigm described by Wera, et al. to be very helpful in the management of the unstable THA. Several studies have shown that tripolar type constrained liners appear to perform considerably better than locking ring type constrained liners. As a result, dual mobility implants are becoming more widely utilised in patients with abductor and other soft tissue deficiencies, hip instability of uncertain etiology and patients with increased risk factors for instability undergoing primary THA. Early results with dual mobility components have been shown to have a low rate of failure in high instability risk revision THAs. These devices do have several unique potential complications and their use should be limited to patients with significantly increased risk of dislocation and instability.

The well-fixed femoral stem can be challenging to remove. Removal of an extensively osteointegrated cementless stem requires disruption of the entire implant-bone interface while a well-fixed cemented stem requires complete removal of all adherent cement from the underlying cortical bone in both the metaphysis and diaphysis of the femur. In these situations, access to those areas of the femur distal to the metaphyseal flare that are beyond the reach of osteotomes and high speed burrs is necessary. This typically requires use of an extended femoral osteotomy (ETO).

The ETO should be carefully planned so that it extends distal enough to allow for access to the end of the stem or cement column and still allow for stable fixation of a new implant. Too short of an ETO increases the risk of femoral perforation by straight burrs, trephines or cement removal instruments that cannot negotiate the bowed femoral canal to access the end of the cement column or end of the stem without risk of perforation. The ETO should also be long enough to allow for fixation with at least 2 cerclage cables. An ETO that is too distal makes implant and cement removal easier, but may not allow for sufficient fixation of a new revision femoral stem. After insertion of the revision stem, the osteotomy is reduced back around the stem and secured in place with cerclage cables.

The well-fixed cemented femoral stem and surrounding cement can be challenging to remove. Success requires evaluation of the quality of the cement mantle (interface lucency), position of the stem, extent of cement below the tip of the stem and skill with the specialised instruments and techniques needed to remove the stem and cement without perforating the femur. Smooth surfaced stems can usually be easily removed from the surrounding cement mantle with a variety of stem extractors that attach to the trunnion or an extraction hole on the implant. Roughened stems can be freed from the surrounding cement mantle with osteotomes or a narrow high speed burr and then extracted with the above instruments. Following this, the well-fixed cement mantle needs to be removed.

Adequate exposure and visualization of the cement column is essential to remove the well-fixed cement without damage to the bone in the femur. This is important since fixation of a revision femoral component typically requires at least 4 cm of contact with supportive cortical bone, which can be difficult to obtain if the femur is perforated or if the isthmus damaged. Proximally, cement in the metaphyseal region can be thinned with a high speed burr, then split radially and removed piecemeal. It is essential to remember that both osteotomes and high speed burrs will cut thru bone easier than cement and use of these instruments poses a substantial risk of unintended bone removal and perforation of the femur if done improperly. These instruments should, as a result, be used under direct vision.

Removal of more distal cement in the femur typically requires use of an extended femoral osteotomy (ETO) to allow for adequate access to the well-fixed cement in the bowed femoral canal. An ETO also facilitates more efficient removal of cement in the proximal femur. The ETO should be carefully planned so that it is distal enough to allow for access to the end of the cement column and still allow for stable fixation of a new implant. Too short of an ETO increases the risk of femoral perforation since the straight cement removal instruments cannot negotiate the bowed femoral canal to access the end of the cement column without risk of perforation. An ETO that is too distal makes cement removal easier, but may not allow for sufficient fixation of a new revision femoral stem. Cement below the level of the ETO cannot be directly visualised and specialised instruments are necessary to safely remove this distal cement. Radiofrequency cement removal devices use high frequency (ultrasonic) radio waves to melt the cement within the canal. Although cement removal with these devices is time consuming and tedious, they do substantially reduce the chances of femoral perforation. These devices can, however, generate considerable heat locally and can result in thermal injury to the bone and surrounding tissues. Once the distal end of the cement mantle is penetrated, backbiting or hooked curettes can be use to remove any remaining cement from within the canal. It is important that all cement be removed from the femur since reamers used for preparation of the distal canal will be deflected by any retained cement, which could result in eccentric reaming and inadvertent perforation of the femur and make fixation of a new implant very challenging. An intra-operative x-ray can be very helpful to insure that all cement has been removed before reaming is initiated.

One should always plan for a possible femoral perforation and have cortical strut grafts and a stem available that will safely bypass the end of the cement column and the previous cement restrictor.

Acute peri-operative blood loss warranting transfusion is a frequent consequence of major joint replacement (TJR) surgery. Significant peri-operative anemia can contribute to hypotension, dyspnea, coronary ischemia and other peri-operative medical events that can result in increased risk of peri-operative complications, readmissions and impair the patient's ability to mobilise after surgery resulting in a longer length of stay (LOS) and increase skilled nursing facility (SNF) utilization. The risks associated with allogeneic blood transfusions (ABT) administered to treat symptomatic peri-operative anemia are numerous and extend beyond the concerns of transmission of communicable disease (HIV, hepatitis, other). Patients receiving ABTs have been shown to have a longer hospital LOS, higher risk of infection, and higher mortality after TJR than those who do not require transfusion after surgery. As a result, many different pre-operative, peri-operative and post-operative strategies have been utilised to minimise peri-operative blood loss and transfusion need for patients undergoing TJR. Several studies have shown that the strongest predictor of the need for ABT in the TJR patient is the pre-operative hematocrit (Hct). As a result, all patients with unexplained pre-operative anemia should be evaluated for an underlying cause prior to elective TJR surgery.

In recent years, focus has shifted towards peri-operative reduction of blood loss with the use of pharmacologic agents like tranexamic acid (TXA). These agents work by inhibiting fibrinolysis and activating plasminogen. Numerous studies have shown that TXA given IV, applied topically into the surgical wound or given orally have been shown to reduce peri-operative bleeding and ABT after both THA and TKR. Regardless of route of administration, all appear to be more efficacious and considerably more cost-effective in reducing the need for ABT than other methods discussed previously. Despite concerns about the potential increased thromboembolic risk in patients undergoing TJR, there does not appear to be any conclusive evidence suggesting an increased risk of venous thromboembolic disease (VTED) in TJR patients who receive peri-operative TXA. Although it may be unnecessary, many TJR surgeons still, however, avoid use of TXA in patients with a past history of VTED, stroke, coronary artery disease (including coronary stents), renal insufficiency, hypercoagulable state and seizure disorder. The use of topical TXA may be safer in some of these high risk patients since systemic absorption is minimal when administered via this route. Although the optimal method of administration (IV, topical, oral or combined) has not yet been determined based on safety, cost and reduction of need for ABT, incorporation of tranexamic acid into a blood conservation program is clearly the standard of care for all TJR programs that should nearly eliminate the need for ABT for patients undergoing TJR.

Utilization of reverse total shoulder arthroplasty (RTSA) has steadily increased since its 2003 introduction in the American market. Although RTSA was originally indicated for elderly, low demand patients, it is now being increasingly used to treat rotator cuff arthropathy, humeral fractures, neoplasms and failed total and hemi shoulder arthroplasty. There is, therefore, a growing interest in bearing surface wear of RTSA polyethylene humeral liners. In the current study, we examined humeral liners retrieved as part of an IRB approved study to determine the amount of bearing surface wear. We hypothesized that wear of the bearing surface by intentional contact with the glenosphere (mode I) would be minor compared to that produced by scapular notching and impingement of the humeral liner (mode II).

Twenty-three retrieved humeral liners were retrieved at revision surgery after an average of 1.5 years implantation time. The average age at implantation was 68 years (range 50–85). Shoulders were revised for loosening (7), instability (6), infection (6), pain (2), and other/unknown reasons (2). The liners were scanned using microCT at a resolution of 50 µm and then registered against unworn surfaces to estimate the bearing surface wear depth. The depth of surface penetration due to impingement of the liner with surrounding structures was measured and the location of the deepest penetration was noted.

Mode I wear of the bearing surface was detectable for five of the retrieved liners. The penetration depth was 100 µm or less for four of the liners and approximately 250 microns for the fifth liner. It was noted that the liners with discernable mode I wear were those with longer implantation times (average 2.4 years). Material loss and abrasion of the rim due to mode II wear was noted with measurable penetration in 18 of the liners. Mode II wear penetrated to the bearing surface in 11 liners. It was generally noted that volumetric material loss was dominated by mode II wear (Figure 1).

In this study of short to medium term retrieved RTSA humeral liners, mode I wear of the bearing surface was a minor source of material loss. Mode II wear due to scapular notching or impingement of the rim was the dominant source of volumetric wear. This is in agreement with a previous study that we have performed on a smaller cohort of seven liners. It is noteworthy that we were able to detect measurable mode I wear for liners with moderate implantation times. The quantity of bearing surface wear that will be seen in long term retrievals remains unknown at this time.

Arthritis of the hip is a relatively common problem in patients with neuromuscular disorders due to muscle imbalance around the hip from weakness, paralysis, contractures and spasticity. Neuromuscular disorders such as cerebral palsy, Parkinson's disease, poliomyelitis, previous cerebrovascular accident (CVA) and Charcot arthropathy have been considered by many to be contraindications to total hip arthroplasty (THA). The presence of certain anatomic abnormalities (excessive femoral anteversion, acetabular dysplasia, leg length discrepancy (LLD) and coax valga) and significant soft tissue contractures, muscle imbalance, and muscular weakness make THA a challenging surgical procedure in this patient population, and can predispose to dislocation and poor functional outcome following surgery. THA can, however, result in substantial pain relief and functional improvement, and can be safely performed, provided certain technical considerations are addressed.

The patient's motor strength and functional status (ambulatory vs. “sitter”) should be carefully assessed preoperatively, since both of these factors may affect the choice of surgical approach and component position. Significant soft tissue contractures should be released at the time of surgery. Although these can be frequently performed “open”, percutaneous adductor tenotomy is occasionally necessary for patients with significant adduction contractures. Patients requiring significant soft tissue releases may benefit from 6 weeks of bracing to allow soft tissues to heal in appropriately and minimise risk of dislocation during this period of time.

Use of modular femoral components that allow for correction of excessive femoral anteversion, should be considered in patients with coexistent dysplasia and neuromuscular disease (i.e., CP or polio). Large femoral head components should also be considered in patients with increased risk factors for dislocation. Despite their obvious theoretical advantages, the use of large head metal on metal THAs should be used with extreme caution in view of growing concerns about these devices. Although constrained acetabular liners are associated with an increased risk of mechanical failure, their use should be strongly considered in patients with significant motor weakness or major soft tissue deficiencies. Meticulous soft tissue closure of the capsule of the hip is recommended, especially when performing THA through a posterior approach. Patients with neuromuscular disorders associated with spasticity and involuntary movements need to be optimally treated medically prior to and indefinitely after THA.

There are limited reports of outcomes following THA in patients with neuromuscular disorders, however, some generalizations based on underlying diagnosis can be made. Patients with cerebral palsy and polio frequently have acetabular dysplasia, excessive femoral anteversion and LLD, and although durability does not seem to be a major concern, dislocation and instability is relatively common and needs to be addressed. Durability and instability do not appear to be major concerns in patients with Parkinson's disease, however, these patients have frequent medical complications perioperatively and have deterioration in function over time due to the progressive nature of their underlying disorder. Patients with previous CVA also appear to have acceptable durability and dislocation risk, but are at high risk of developing heterotopic ossification postoperatively. Patients with Charcot arthropathy or myelodysplasia are at high risk of instability and appear to have limited functional improvement following THA. As a result, the consensus of opinion is that THA is contraindicated in these patients.

Numerous studies have shown highly cross-linked polyethylene (XLPE) to be an extremely low wear bearing surface for total hip arthroplasty (THA) at intermediate term follow-up. Wear rates and the incidence of osteolysis for CoCr femoral heads on XLPE liners appears to be considerably less than what is observed for conventional polyethylene (PE). This has been demonstrated even in younger, more active patients.

Nevertheless, polyethylene wear and associated osteolysis are still a concern, since the indications for THA have been expanded to include younger and more active patients. Both wear simulator and clinical data suggest that ceramic femoral heads can reduce bearing surface wear of conventional PE. There is, however, extremely limited evidence supporting any advantage of ceramic femoral heads over CoCr femoral heads with regards to bearing surface wear of XLPE. This is perhaps due to the relative difficulty in measuring the low wear rates of XLPE bearings in general, regardless of material composition of the femoral head. Although ceramic femoral heads are more scratch resistant and less susceptible to third body wear, their current clinical use to reduce wear of XLPE bearings is, in reality, based on the unproven assumption that use of ceramic femoral heads will have a similar effect on wear reduction as is seen with ceramic on conventional PE bearing couples. Nevertheless, the use of ceramic femoral heads has become common in younger, more active patients.

Recently, corrosion at the head neck junction of modular THA (trunnionosis), has been determined to be the possible source of metal debris and metal ions associated with adverse local tissue reactions (ALTR or ARMD) in THA, including ALVAL and pseudotumors. There is general agreement that trunnionosis results from mechanically assisted crevice corrosion (fretting) of the modular junctions common to nearly all contemporary THA designs. Several design, material and patient factors have been implicated as contributors to this problem including larger diameter femoral heads (>36 mm), reduced femoral neck and taper geometry, flexural rigidity of the taper, and patient body weight and activity level. Data from our multicenter implant retrieval program has shown that corrosion at the head-neck junction of contemporary modular THAs may be reduced with use of ceramic femoral heads. The use of ceramic femoral heads also eliminates the potential for release of cobalt and chromium ions from the taper junctions of titanium alloy stems. In younger patients, the long term effects of cobalt ions released from corrosion at the modular neck junction are still unknown.

Although the surgeon's selection of a ceramic femoral head in combination with a XLPE acetabular liner is likely based on the desire to minimise PE wear, the impact of femoral head composition on taper neck corrosion and ALTR is perhaps more of a concern in 2015. Until the problem of taper neck corrosion is more thoroughly understood and effectively addressed by implant manufacturers, the use of ceramic femoral heads in THA should be considered in the younger or more active patient. The increased cost of ceramic femoral heads creates a dilemma in defining who is “young” enough and “active” enough to be considered an appropriate candidate for a ceramic femoral head in our current environment of bundled care payments, value based purchasing and concern about providing cost-effective health care to our patients.

Introduction

Highly crosslinked polyethylene (HXLPE) was clinically introduced approximately a decade and a half ago to reduce polyethylene wear rates and subsequent osteolysis. Clinical and radiographic studies have repeatedly shown increased wear resistance, however concerns of rim oxidation and fatigue fracture remain. Although short to intermediate term retrieval studies of these materials are available, the long-term behavior of these materials remains unclear.

Introduction

Previous studies of retrieved CoCr alloy femoral heads have identified imprinting of the stem taper surface features onto the interior head bore, leading researchers to hypothesize that stem taper microgrooves may influence taper corrosion. However, little is known about the role of stem taper surface morphology on the magnitude of in vivo corrosion damage. We designed a matched cohort retrieval study to examine this issue.

INTRODUCTION

Mechanically assisted crevice corrosion of taper interfaces was raised as a concern in total hip arthroplasty (THA) approximately 20 years ago (Gilbert 1993). In total shoulder replacement, however, comparatively little is known about the prevalence of fretting assisted crevice corrosion or the biomechanical and patient factors that influence this phenomenon. Given the comparatively lower loading experienced in the shoulder compared to the hip, we asked: (1) What is the prevalence of fretting assisted corrosion in modular total shoulder replacements, and (2) What patient and implant factors are associated with corrosion?

INTRODUCTION

Retrieval analysis is an important aspect of medical device development. Examination of retrieved devices allows device developers to close the design loop, understand the performance of devices, and validate assumptions made and methods used during preclinical testing. We provide an overview of the implant retrieval analysis performed at the Implant Research Center at Drexel University on reverse total shoulder systems retrieved after short to medium term implantation.

Introduction

A variety of porous coatings and substrates have been used to obtain fixation at the bone-implant interface. Clinical studies of porous tantalum, have shown radiographically well-fixed implants with limited cases of loosening. However, there has been limited retrieval analysis of porous tantalum hip implants. The purpose of this study was to investigate factors affecting bone ingrowth into porous tantalum hip implants.

Introduction

Recent implant design trends have renewed concerns regarding metal wear debris release from modular connections in THA. Previous studies regarding modular head-neck taper corrosion were largely based on cobalt chrome (CoCr) alloy femoral heads. Comparatively little is known about head-neck taper corrosion with ceramic femoral heads or about how taper angle clearance influences taper corrosion. This study addressed the following research questions: 1) Could ceramic heads mitigate electrochemical processes of taper corrosion compared to CoCr heads? 2) Which factors influence stem taper corrosion with ceramic heads? 3) What is the influence of taper angle clearance on taper corrosion in THA?

Introduction

The release of metal debris and ions has raised concerns in joint arthroplasty. In THA metal debris and ions can be generated by wear of metal-on-metal bearing surfaces and corrosion at modular taper interfaces, currently understood to be mechanically assisted crevice corrosion (MACC) [1]. More recently, inflammatory-cell induced corrosion (ICIC) has been identified as a possible source of metal debris and/or ions [2]. Although MACC has been shown to occur at modular junctions in TKA, little is known about the prevalence of other sources. The purpose of this study was to determine the sources of metallic debris and ion release in long-term implanted (in vivo > 15y) TKA femoral components. Specific attention was paid to instances of ICIC as well as damage at the implant-bone interface.

The well-fixed cemented femoral stem and surrounding cement can be challenging to remove. Success requires evaluation of the quality of the cement mantle (interface lucency), position of the stem, extent of cement below the tip of the stem and skill with the specialised instruments and techniques needed to remove the stem and cement without perforating the femur.

Smooth surfaced stems can usually be easily removed from the surrounding cement mantle with a variety of stem extractors that attach to the trunnion or an extraction hole on the implant. Roughened stems can be freed from the surrounding cement mantle with osteotomes or a narrow high speed burr and then extracted with the above instruments. Following this, the well fixed cement mantle needs to be removed.

Adequate exposure and visualization of the cement column is essential to remove the well-fixed cement without damage to the bone in the femur. This is important since fixation of a revision femoral component typically requires at least 4cm of contact with supportive cortical bone, which can be difficult to obtain if the femur is perforated or if the isthmus damaged. Proximally, cement in the metaphyseal region can be thinned with a high speed burr, then split radially and removed piecemeal. It is essential to remember that both osteotomes and high speed burrs will cut thru bone easier than cement and use of these instruments poses a substantial risk of unintended bone removal and perforation of the femur if done improperly. These instruments should, as a result, be used under direct vision.

Removal of more distal cement in the femur typically requires use of an extended femoral osteotomy (ETO) to allow for adequate access to the well-fixed cement in the bowed femoral canal. An ETO also facilitates more efficient removal of cement in the proximal femur. The ETO should be carefully planned so that it is distal enough to allow for access to the end of the cement column and still allow for stable fixation of a new implant. Too short of an ETO increases the risk of femoral perforation since the straight cement removal instruments cannot negotiate the bowed femoral canal to access the end of the cement column without risk of perforation. An ETO that is too distal makes cement removal easier, but may not allow for sufficient fixation of a new revision femoral stem. Cement below the level of the ETO cannot be directly visualised and specialised instruments are necessary to safely remove this distal cement. Radiofrequency cement removal devices (OSCAR) use high frequency (ultrasonic) radio waves to melt the cement within the canal. Although cement removal with these devices is time consuming and tedious, they do substantially reduce the chances of femoral perforation. These devices can, however, generate considerable heat locally and can result in thermal injury to the bone and surrounding tissues. Once the distal end of the cement mantle is penetrated, backbiting or hooked curettes can be used to remove any remaining cement from within the canal. It is important that all cement be removed from the femur since reamers used for preparation of the distal canal will be deflected by any retained cement, which could result in eccentric reaming and inadvertent perforation of the femur and make fixation of a new implant very challenging. An intraoperative x-ray can be very helpful to insure that all cement has been removed before reaming is initiated.

Protrusio acetabuli (arthrokatadysis or Otto pelvis) is a relatively rare condition associated with secondary osteoarthritis of the hip. Radiographically, protrusio acetabuli is present when the medial aspect of the femoral head projects medial to Kohler's (ilioischial) line. This results in medialization of the center of rotation (COR) of the hip. Protrusio acetabuli is typically associated with metabolic bone disease (osteoporosis, osteomalacia, Paget's disease) or inflammatory arthritis (RA or ankylosing spondylitis). Idiopathic acetabular protrusio can occur without the above associated factors however. Patients with protrusio acetabuli typically present with significant restriction of range of motion (ROM) of the hip due to femoral neck and trochanteric impingement in the deep acetabular socket and pain associated with secondary osteoarthritis (OA).

Total hip arthroplasty (THA) in patients with protrusion acetabuli is more challenging than THA in patients with a normal hip COR. ROM is typically quite restricted which can compromise surgical exposure. Dislocation of the hip in the patient with a deep socket and medialised COR can be extremely difficult and associated with fracture of the femur if not carefully performed. Restoration of the hip COR to the normal more lateralised position is a principle goal of surgery. This restores more normal mechanics of the hip and has been associated with improved durability. A variety of techniques to accomplish this have been described including medial acetabular bone grafting with cemented cups, protrusio rings or porous coated cementless cups fixed with multiple screws. The latter technique has been shown to be more durable and associated with better outcomes.

THA in protrusio acetabuli starts with templating of the preoperative x-rays to determine the optimal acetabular implant size and final position of the acetabular component that restores the hip COR to the normal position. Patients with protrusio acetabuli often have varus oriented femoral necks and the femur needs to be carefully templated as well to insure that an appropriate femoral component is available that will allow for restoration of the patient's anatomy. Cartilage covering the thinned medial wall needs to be carefully removed without disruption of the medial acetabular wall. The acetabulum is then carefully reamed with the goal of obtaining stable peripheral rim support of a cementless socket and at least 50% contact of the implant on good quality host bone. Unlike acetabular preparation in the normal hip, preventing the reamer from “bottoming out” is essential in order to obtain desired rim support and return of the hip COR to the normal lateralised position. When good rim support of the reamer is obtained, a trial component is placed and intraoperative x-ray obtained to confirm fit, position and restoration of hip COR. Limited addition reaming can be performed to obtain desired degree of press fit (1‐2mm) and contact with host bone. Morselised autograft from the femoral head and neck is then packed into the medial defect and reverse reamed. The cementless acetabular component is then impacted into position and fixed with screws. Weight bearing is determined by bone quality, size and containment of the medial defect, amount of contact of the cementless cup with host bone and stability of the acetabular construct. Incorporation of autograft bone in the acetabulum and stable long term fixation occurs reliably if stable initial press-fit fixation of the cementless cup is obtained. Restoration of hip COR to within 7mm of its normal location is associated with better implant survival.

Summary Statement

This study assesses oxidation, mechanical behavior and revision reasons of 2^nd generation HXLPE used in total hip and knee arthroplasty. While oxidation was low for both X3 and E1 HXLPEs, oxidative regional variations were detected in the sequentially annealed cohort.

Summary Statement

Fretting and corrosion has been identified as a clinical problem in modular metal-on-metal THA, but remains poorly understood in modern THA devices with polyethylene bearings. This study investigates taper damage and if this damage is associated with polyethylene wear.

Arthritis of the hip is a relatively common problem in patients with neuromuscular disorders due to muscle imbalance around the hip from weakness, paralysis, contractures and spasticity. Neuromuscular disorders such as cerebral palsy, Parkinson's disease, poliomyelitis, previous cerebrovascular accident (CVA) and Charcot arthropathy have been considered by many to be relative contraindications to total hip arthroplasty (THA). The presence of certain anatomic abnormalities (excessive femoral anteversion, acetabular dysplasia, leg length discrepancy (LLD) and coax valga) and significant soft tissue contractures, muscle imbalance, and muscular weakness make THA a challenging surgical procedure in this patient population, and can predispose to dislocation and poor functional outcome following surgery. THA can, however, result in substantial pain relief and functional improvement in patients with significant hip arthritis and neuromuscular disorders, and can be safely performed, provided certain technical considerations are addressed.

The patient's motor strength and functional status (ambulatory vs. “sitter”) should be carefully assessed pre-operatively, since both of these factors may affect the choice of surgical approach and component position. Significant soft tissue contractures should be released at the time of surgery. Although these can be frequently performed “open”, percutaneous adductor tenotomy is occasionally necessary for patients with significant adduction contractures. Patients requiring significant soft tissue releases may benefit from 6 weeks of bracing to allow soft tissues to heal appropriately and minimise risk of dislocation during this period of time.

Use of modular femoral components that allow for correction of excessive femoral anteversion, should be considered in patients with coexistent dysplasia and neuromuscular disease (i.e. CP or polio). Large femoral head components should also be considered in patients with increased risk factors for dislocation. Despite their obvious theoretical advantages, the use of large head metal on metal THAs should be used with extreme caution in view of growing concerns about these devices. Although constrained acetabular liners are associated with an increased risk of mechanical failure, their use should be strongly considered in patients with significant motor weakness or major soft tissue deficiencies. Meticulous soft tissue closure of the capsule of the hip is recommended, especially when performing THA through a posterior approach. Patients with neuromuscular disorders associated with spasticity and involuntary movements need to be optimally treated medically prior to and indefinitely after THA.

There are limited reports of outcomes following THA in patients with neuromuscular disorders, however some generalisations based on underlying diagnosis can be made. Patients with cerebral palsy and polio frequently have acetabular dysplasia, excessive femoral anteversion and LLD, and although durability does not seem to be a major concern, dislocation and instability is relatively common and needs to be addressed. Durability and instability do not appear to be major concerns in patients with Parkinson's disease, however, these patients have frequent medical complications perioperatively and have deterioration in function over time due to the progressive nature of their underlying disorder. Patients with previous CVA also appear to have acceptable durability and dislocation risk, but are at high risk of developing heterotopic ossification postoperatively. Patients with Charcot arthropathy or myelodysplasia are at high risk of instability and appear to have limited functional improvement following THA. As a result, the consensus of opinion is that THA is contraindicated in patients with Charcot arthropathy and myelodysplasia.

Revision TKR is a challenging surgical procedure that requires considerable pre-operative evaluation and planning. The diagnostic evaluation for the presence of periprosthetic sepsis has been well described and is of paramount importance. Optimal results of revision TKR mandate that the etiology of failure, and reason for revision, be clearly understood, since the outcome of revision TKR for unexplained pain has been disappointing.

Physical examination should include a careful assessment of range of motion, ligament stability, quality of the soft tissues around the knee and the location of any prior incisions around the knee. The above information provides valuable information about any potential difficulties with surgical exposure, selection of the safest surgical incision, potential problems with soft tissue coverage and selection of an implant with the proper degree of constraint.

Plain radiographs should be carefully evaluated for fixation, alignment, osteolysis and extent of bone deficiency around each component. This information is essential in order to develop a strategy for removal of existing implants and cement, obtaining satisfactory fixation of a new implant and managing bone deficiencies encountered at the time of surgery using a variety of stems, augments and bone grafts.

The goals of revision TKR are simple to state but difficult to obtain: stable implant fixation, a healed surgical wound without infection, restoration of alignment, stability and a functional range of motion. Despite the most conscientious pre-operative planning, one must be prepared for the unexpected and any surgical plan requires a sound “back-up” plan.