Aims

Hip arthroplasty does not always restore normal anatomy. This is due to inaccurate surgery or lack of stem sizes. We evaluated the aptitude of four total hip arthroplasty systems to restore an anatomical and medialized hip rotation centre.

Evaluating musculoskeletal conditions of the lower limb and understanding the pathophysiology of complex bone kinematics is challenging. Static images do not take into account the dynamic component of relative bone motion and muscle activation. Fluoroscopy and dynamic MRI have important limitations. Dynamic CT (4D-CT) is an emerging alternative that combines high spatial and temporal resolution, with an increased availability in clinical practice. 4D-CT allows simultaneous visualization of bone morphology and joint kinematics. This unique combination makes it an ideal tool to evaluate functional disorders of the musculoskeletal system. In the lower limb, 4D-CT has been used to diagnose femoroacetabular impingement, patellofemoral, ankle and subtalar joint instability, or reduced range of motion. 4D-CT has also been used to demonstrate the effect of surgery, mainly on patellar instability. 4D-CT will need further research and validation before it can be widely used in clinical practice. We believe, however, it is here to stay, and will become a reference in the diagnosis of lower limb conditions and the evaluation of treatment options.

Cite this article: Bone Joint J 2021;103-B(5):822–827.

Introduction

Although total hip arthroplasty is a very successful operation, complications such as: dislocation, aseptic loosening, and periprosthetic fracture do occur. These aspects have been studied in large populations for traditional stem designs, but not for more recent short stems. The design rationale of short stems is to preserve bone stock, without compromising stability. However, due to their smaller bone contact area, high peak stresses and areas of stress shielding could appear in the proximal femur, especially in the presence of atypical bone geometries. In order to evaluate this aspect, we quantified the stress distribution in atypical proximal femurs implanted with a commercially available calcar guided short stem.

Introduction

Natural population variation in femoral morphology results in a large range of offsets, anteversion angles and lengths. During total hip arthroplasty, accurate restoration of hip biomechanics is essential to achieve good functional results. One option is to restore the anatomic hip rotation center. Alternatively, medializing the rotation center and compensating by increasing the femoral offset, reduces acetabular contact forces and increases the abductor lever arm. We investigated the ability of two cemented stem systems to restore hip biomechanics in an anatomic and medialized way. We compared an undersized “Exeter-type” of stem with three offset options and 18 sizes (CPT, Zimmer), to a line-to-line “Kerboul-type” of stem with proportional offset and 12 sizes (Centris, Mathys).

Accurate detection of migration of hip arthroplasty stems without the burden of bone markers and stereo-radiographic equipment is of interest. This would facilitate the study of stem migration in an experimental setting, but more importantly, it would allow assessing stem loosening in patients with a painful hip outside a study protocol.

We developed and validated a marker-free automated CT-based spatial analysis method (CTSA) to quantify stem-bone migration in successive CT scan acquisitions. First, we segmented the bone and stem within both three-dimensional images, then we pairwise registered those elements (Fig. 1). By comparing the rigid transformations of stem and bone, we calculated the migration of the stem with reference to the bone and transferred the three translation and three rotation parameters to an anatomic coordinate system. Based on the rigid transformation, we also calculated the point of the stem that presented the maximal migration (PMM).

Accuracy was assessed in a stem-bone model (Fig. 2) by imposing 39 predefined stem rotations and translations, and by comparing those with values calculated with the CTSA tool. In all cases, differences were below 0.20 mm for translations and 0.19° for rotations (95% tolerance interval (95% TI) below 0.22 mm and 0.20°, largest standard deviation of the signed error (SDSE) 0.081 mm and 0.057°). Precision was defined as stem migration calculated in eight clinical relevant zero-migration scenarios. In all cases, precision was below 0.05 mm and 0.08° (95% TI below 0.06 mm and 0.08°, largest SDSE 0.012 mm and 0.020°). The largest displacement of the PMM on the stem was 0.169mm. The precision estimated in five patients was very dependent on the CT scan resolution and was below 0.48 mm and 0.37° (95% TI below 0.59 mm and 0.61°, largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.972 mm). In optimized conditions, the precision in patients improved largely and was below 0.040 mm and 0.111° (largest SDSE 0.202 mm and 0.279°, largest displacement of the PMM 0.156 mm).

Our marker-free automated CT-based spatial analysis can detect hip stem migration with an accuracy and precision comparable to that of radiostereometric analysis (RSA), but without the burden of bone markers and the cost of stereo-radiographic equipment. As such, we believe our tool could make accurate measurement of stem migration available to departments without access to RSA and boost this type of research. Moreover, as CTSA does not rely on bone makers, it is applicable to all-comers with a painful hip arthroplasty. Indeed, in those patients with a reference CT scan after hip replacement, a new CT scan could demonstrate stem migration. If no initial CT scan is available, a reference scan could be taken during a first visit and repeated later. Additionally, a “stress test” of the hip could be performed. During such test, comparing CT images acquired during forced maximal intern and external rotation could demonstrate stem loosening.

INTRODUCTION

Thermal necrosis of the femoral head, due to heat generation during cement polymerization, is a concern in hip resurfacing. Bone necrosis could cause fractures and/or implant loosening. Some authors¹ found an inverse relationship between the size of the femoral component and the risk of revision after hip resurfacing. We postulate that smaller implants contain proportionally more cement than larger ones and that this could explain the effect of implant size on revision rate. As such, we investigated the relation between implant size and both, the average cement mantle thickness and the cement-filling index (fraction of cement volume and total volume within the implant).

Introduction

Hip resurfacing arthroplasty has gained popularity as an alternative for total hip arthroplasty. Usually, cemented fixation is used for the femoral component. However, each type of resurfacing design has its own recommended cementing technique.

In a recent investigation the effect of various cementing techniques on cement mantle properties was studied. This study showed distinct differences in cement mantle volume, filling index and morphology.

In this study, we investigated the effect of these cement mantle variations on the heat generation during polymerization, and its consequences in terms of thermal bone necrosis.

Introduction: It is generally accepted that the cement mantle surrounding femoral hip implants should be at least 2–3 mm thick. To achieve that goal, manufactures or surgeons often undersize the stem compared to the broach. However, some implants, such as the Charnley-Kerboul stem, are typically cemented line-to-line i.e. with a broach and stem of the same size. Despite their “minimal” cement mantle, these stems are very successful. This apparent contradiction is known as the “French Paradox”[1]. We used a finite element analysis (FEA) model to investigate the effect of these different cementation philosophies on cement crack propagation and rotational stem stability.

Material and Methods: Based on a CT-scan image of a Charnley-Kerboul plastic stem replica[2], twelve FEA models were created. By decreasing the stem size (4 stems), the average cement mantle thickness increased (1.71–3.77mm). However, the incidence of cement mantle defects (< 1mm) and areas of thin cement (< 2mm) decreased (defects: 34.7–0.0%; thin cement: 40.7–0.0%). The amount of cortical bone support was varied (3 times) between 18.4 and 72.2%. All models were alternately loaded with a cyclic torque load (25.8Nm) and a transversal load (400N) in a ratio of 9:1 for two million cycles. The model predicted fatigue crack formation within the cement and rotational stem stability.

Results: Overall, increasing implant size and increasing the amount of cortical bone support to the cement, improved resistance to accumulated cement damage and rotational stem stability. In both models with undersized stems, more cement cracks and full thickness (FT) cement fractures appeared after less loading cycles than in both models with canal-filling stems. Worst results were obtained with a severely undersized implant surrounded by a thick cement mantle that was poorly supported by cortical bone (first FT crack after < 100 000 cycles, > 220 initiated cracks and 0.6° of implant rotation after 2 million cycles). Best results were obtained with the maximal canal-filling stem surrounded by a thin and deficient cement mantle that was well supported by cortical bone (no FT cracks, < 10 initiated cracks and 0.3° of implant rotation after 2 million cycles).

Conclusion: This study emphasizes the importance of an adequate cementation technique that aims at pressurizing cement up to the cortical bone. This protects the cement mantle against fatigue fracture and stabilises the implant especially if the stem is undersized. From a mechanical point of view, canal-filling stems make sense. They limited the formation of cement cracks and improved rotational stability to the implant. This could explain the excellent results obtained by implants that are cemented line-to-line.

Background: Interfacial gaps between cement and femoral hip implants are a potential source of stem debounding and loosening. We used a CT-scan technique to measure the characteristics of these gaps for two different implant types and two different implantation protocols.

Methods: Using a third generation cementing technique, 22 plastic replicas of straight Charnley-Kerboul stems (Stratec) and 18 replicas of anatomic Lubinus SPII stems (Waldemar-Link) were implanted in 20 pairs of embalmed cadaver femora. In each pair of femora, the same stem type was used. However, at one side a stem with the same size as the broach was used (line-to-line), while at the other, we cemented a stem that was one size smaller (undersized). Based on a validated CT-scan measurement tool[1], we quantified the extent of interfacial gaps, determined their location and measured the cement thickness in areas with and without gaps. Differences in interfacial gaps between both implant types and both implantation techniques were analysed with a general linear model (GLM).

Results: In total 10308 connective CT-images were segmented and analysed. Interfacial gaps were found in every specimen (average: 6.43% of the stem surface), but their extent varied widely between specimens (SD: 8.99%; 0.15% – 31.69%). According to a GLM, the quantity of interfacial gaps did not vary significantly between implant types (Kerboul: 7.92±10.69%; Lubinus: 4.61±6.14%, p=0.246) and between implantation techniques (line-to-line: 7.73±10.24%; undersized: 5.13±7.57%, p=0.416). Irrespective of the implant type and the implantation protocol, flat regions of the stem surface showed significantly more interfacial gaps than corner regions (flat: 6.02%; corner: 4.36%; Chi-square: p< 0.001). Overall, more gaps were found at the anterior surface of the implant (anterior: 7.61%; medial: 4.99%; lateral: 4.46%; posterior: 4.43%; Chi-square: p< 0.001). For Kerboul and line-to-line stems, the extent of interfacial gaps increased consistently from the distal to the proximal stem region. This was not true for Lubinus and undersized stems. The thickness of the cement mantle showed no significant differences in areas with and without interfacial gaps (cement thickness: 3.50 mm in gap regions and 3.45 mm in regions without gaps, paired t-test: p=0.823).

Conclusions: Interfacial gaps between a femoral hip implant and cement are common and were found in all specimens. As these gaps can be extensive (up to 30 % of the stem surface), they could influence implant stability and survival. The interfacial gaps described here were probably due to air that was introduced along the implant during stem insertion. Further studies are needed to confirm that theory, to determine the consequences of these gaps and to search for methods to prevent them.

Interfacial defects between the cement mantle and a hip implant may arise from constrained shrinkage of the cement or from air introduced during insertion of the stem. Shrinkage-induced interfacial porosity consists of small pores randomly located around the stem, whereas introduced interfacial gaps are large, individual and less uniformly distributed areas of stem-cement separation. Using a validated CT-based technique, we investigated the extent, morphology and distribution of interfacial gaps for two types of stem, the Charnley-Kerboul and the Lubinus SPII, and for two techniques of implantation, line-to-line and undersized.

The interfacial gaps were variable and involved a mean of 6.43% (sd 8.99) of the surface of the stem. Neither the type of implant nor the technique of implantation had a significant effect on the regions of the gaps, which occurred more often over the flat areas of the implant than along the corners of the stems, and were more common proximally than distally for Charnley-Kerboul stems cemented line-to-line. Interfacial defects could have a major effect on the stability and survival of the implant.

We undertook a review of the literature relating to the two basic stem designs in use in cemented hip replacement, namely loaded tapers or force-closed femoral stems, and the composite beam or shape-closed designs. The associated stem fixation theory as understood from in vitro studies and finite element modelling were examined with reference to the survivorship results for each of the concepts of fixation.

It is clear that both design principles are capable of producing successful long-term results, providing that their specific requirements of stem metallurgy, shape and surface finish, preparation of the bone and handling of the cement are observed.

Software to segment and to analyse connective CT-scan images of the bone-cement-stem complex was developed and validated. Parameters assessed included: volumes (cortical bone, cancelous bone, cement, stem, air in bone and air in cement), cement mantle thickness, cortical & cancelous bone thickness, contact surface area between cement and bone, degree of centralisation (stem in cement, stem and cement in cancelous and in cortical bone). To validate and assess intra- and interob-server reliability two models were implanted in two dried macerated cadaver femurs using a third generation cementing technique. In the first a polished tapered stem (CPT, Zimmer) was cemented and removed after cement curing. The air filled cavity within the cement mantle could be identified as implant, avoiding metallic scatter artefacts. The second model (SLA) used a plastic stem prototype produced by computer design and a rapid prototyping stereolithographic technique. This model does not need to be removed before CT-scanning and allows assessment of whatever femoral implant. Validation occurred by comparing 41 manually segmented femoral cross-sections (25 CPT, 16 SLA) with data of corresponding CT-scan slices. Inter-observer reliability was assessed by having the same person performing the CT-scan and the analysis of both models four times. To assess intra-observer reliability, four different observers segmented 97 representative CT-images (46 CPT, 51 SLA). The average accuracy for surfaces areas (bone, cement, stem) within CT-images was 7.47 mm2 (1.80%), bone & cement mantle thickness: 0.51 mm (9.39%), distances between centroids (stem-cement, stem-bone, cement-bone): 0.38 mm (18.5%) and contours (bone, cement): 0.27 mm (2.57%). The intra- and interobserver reliability of air content in bone and cement was suboptimal (intraclass-correlation coefficient (ICC) as low as 0.54, average ICC: 0.85). All other variables assessed were reliable (ICC > 0.81, average ICC: 0.96). Validity and reliability were comparable when assessed separately for the proximal, middle and distal third of both models. This in vitro technique can assess characteristics of cement mantles produced by different cementing techniques, centralizers and existing femoral implants or stem prototypes.

During a one-year period starting in October 2001, we analysed the intra-hospital cost of 102 primary elective total hip arthroplasty (THA) in a Belgian university hospital. Patients were treated according to age and general condition with an all cemented metal-poly THA (37), a uncemented cup and cemented metal-poly THA (40), an all uncemented ceramic-ceramic THA (18) or non-standard implants or combinations (7). On average patients stayed 14.4 days in the orthopaedic ward and intra-hospital cost was 9496 Euro (SD: 2178): 53.8% was related to hospitalisation, 21.3% to implants and material, 7.7% to surgery and 4.1% to anaesthesia. A multiple regression analysis was performed to identify possible influencing factors for intra-hospital cost and stay (pre-operative hip function, general health and dwelling as well as implant choice and intra-hospital complications). Overall, only the occurrence of complications during hospitalisation had a significant regression coefficient. In total 14 patients (13.7%) suffered at least one complication during hospitalization (dislocation: 4.9%, heamatoma or superficial infection: 2%, trochanter fracture: 1%, thrombosis with pulmonary embolism: 1%, general complications: 6.9%). This resulted in a significant higher cost (11823 versus 9125 Euro) and hospital stay (19.4 versus 13.6 days). For those patients who did not suffer complications, only implant choice and the place patients were discharged to had significant regression coefficients. The average implant cost for cemented metal-poly THA was 1444 Euro (16.1% of the total cost) compared to 2686 Euro (25.6% of the total cost) for uncemented ceramic-ceramic implants. Due to a chronic shortage of rehabilitation units in the Brussels region, discharged to these units led to both higher cost (10422 versus 9056 Euro) and longer hospital stay (16.5 versus 13.4 days). In the Belgian health insurance system, limitation of intra-hospital cost can best be achieved by shortening hospital stay after THA. This might include improved control of postoperative complications, faster rehabilitation programs and improved surgical techniques to reduce the needs for rehabilitation units and to allow earlier return to independency. Another option is to increase cost awareness regarding prolonged hospital stay of both, patients and medical staff.

Using a modern cementing technique, we implanted 22 stereolithographic polymeric replicas of the Charnley-Kerboul stem in 11 pairs of human cadaver femora. On one side, the replicas were cemented line-to-line with the largest broach. On the other, one-size undersized replicas were used (radial difference, 0.89 mm sd 0.13).

CT analysis showed that the line-to-line stems without distal centralisers were at least as well aligned and centered as undersized stems with a centraliser, but were surrounded by less cement and presented more areas of thin (< 2 mm) or deficient (< 1 mm) cement. These areas were located predominantly at the corners and in the middle and distal thirds of the stem. Nevertheless, in line-to-line stems, penetration of cement into cancellous bone resulted in a mean thickness of cement of 3.1 mm (sd 0.6) and only 6.2% of deficient and 26.4% of thin cement. In over 90% of these areas, the cement was directly supported by cortical bone or cortical bone with less than 1 mm of cancellous bone interposed.

When Charnley-Kerboul stems are cemented line-to-line, good clinical results are observed because cement-deficient areas are limited and are frequently supported by cortical bone.

The retrograde Marchetti-Vicenzi humeral nail consists of four or five flexible branches. At one end these branches are fixed into a solid L-shaped cylinder and at the other, they are held together with a locking wire. The nail is inserted in a retrograde way through a cortical window proximal to the olecranon fossa. Once passed the fracture, removing the locking wire allows the branches to spread in the metaphysis providing proximal stability. Distal locking is achieved through screw fixation.

The Marchetti-Vicenzi nail presents several theoretical advantages. Its flexible branches facilitate nail insertion and might favour fracture healing. Distal locking is performed under direct vision from posterior to anterior and additional proximal locking is not required, preventing iatrogenic neurovascular damage. Distal locking avoids nail migration and retrograde nail insertion spares the rotator cuff. Early mobilisation is often possible. This leads to an equally good elbow function, but with a better shoulder function compared to antegrade nailing.

On the other hand, limited rotational stability, especially in transverse fractures, can cause non-union and hardware failure. In the initial design, the bulky L-shaped end made a large supracondylar insertion and removal window mandatory, increasing the risk of fracture in this area. In the later version, the angle of the L-shaped cylinder has therefore been modified.

In our opinion, the use of the Marchetti-Vicenzi nail is not advisable in comminuted or transverse humeral fractures, in fractures extending in the distal third or in patients (young ladies) with a narrow medullary canal. Nail removal should only be considered if absolutely necessary.

We carried out percutaneous, arthroscopically- and fluoroscopically-assisted osteosynthesis of fractures of the tibial plateau in 52 patients, of whom 38 were assessed using the HSS knee score and standing radiographs. We reviewed 31 AO type-B fractures and seven type-C fractures after a mean follow-up of five years (1 to 14). Fixation was achieved using percutaneous screws and/or an external frame; 33 associated intra-articular injuries, diagnosed in 21 out of the 38 patients, were treated arthroscopically.

Subjectively, 94.7% of the patients reviewed were satisfied. According to the HSS knee score 78.9% of the results were excellent, 13.2% good, 7.9% fair and none was poor. Narrowing of the joint space was found in 28.9% of the injured and 5.3% of the unaffected knees and axial deviation of 5° to 10° in 15.8% of the injured and 10.5% of the unaffected knees.

Of the 52 fractures, reduction was incomplete in one, and in two secondary displacement occurred, of which one required corrective osteotomy. Deep-venous thrombosis occurred in four cases.

The technique has proved to be safe but demanding. It facilitates diagnosis and appropriate treatment of associated intra-articular lesions.