Background

Total Ankle Replacement (TAR) has become a common surgical procedure for severe Osteoarthritis of the ankle. Unlike hip and knee, current TARs still suffer from high failure rates. A key reason could be their non-anatomical surface geometry design, which may produce unnatural motion and load-transfer characteristics. Current TARs have articular surfaces that are either cylindrical or truncated cone surfaces following the Inman truncated cone concept from more than 60 years ago [1]. Our recent study demonstrated, that the surfaces of the ankle can be approximated by a Saddle-shaped, Skewed, truncated Cone with its apex directed Laterally (SSCL) [2]. This is significantly different than the surface geometry used in current TAR systems. The goal of this study was to develop and test the reliability of an in vitro procedure to investigate the effect of different joint surface morphologies on the kinematics of the ankle and to use it to compare the effect of different joint surface morphologies on the 3D kinematics of the ankle complex.

Introduction

Total knee arthroplasty (TKA) is a consolidated orthopaedic procedure and success of such operation depends on the prosthetic design [1]. Unfortunately, as there is a good survival rate of primary TKA, failures occur for factors concerning the polyethylene composition of the implants, secondary osteolysis, and ultimately loosening of the implants are the usual causes of failure after normal use [2]. Dynamic in vitro testing of the human knee continues to be an area of interest to the orthopaedic biomechanics community. The scope of this work was to assess pre-clinically the wear behaviour of polyethylene knee insert under a realistic stair climbing activity using a displacement knee simulator.

INTRODUCTION

In total knee arthroplasty (TKA), the effectiveness of the mechanical alignment (MA) within 0°±3° has been recently questioned. A novel implantation approach, i.e. the kinematic alignment (KA), emerged recently, this being based on the pre-arthritic lower-limb alignment. In KA, the trans-cylindrical axis is used as the reference, instead of the trans-epicondylar one, for femoral component alignment. This axis is defined as the line passing through the centres of the posterior femoral condyles modeled as cylinders. Recently, patient specific instrumentation (PSI) has been introduced in TKA as an alternative to conventional instrumentation. This provides a tool for preoperative implant planning also via KA. Particularly, KA using PSI seems to be more effective in restoring normal joint kinematics and muscle activity.

The purpose of this study was to report preliminarily joint kinematic and electromyography results of two patient groups operated via conventional MA or KA, the latter using PSI.

INTRODUCTION

In computer-aided total knee arthroplasty (TKA), surgical navigation systems (SNS) allow accurate tibio-femoral joint (TFJ) prosthesis implantation only. Unfortunately, TKA alters also normal patello-femoral joint (PFJ) functioning. Particularly, without patellar resurfacing, PFJ kinematics is influenced by TFJ implantation; with resurfacing, this is further affected by patellar implantation. Patellar resurfacing is performed only by visual inspections and a simple calliper, i.e. without computer assistance.

Patellar resurfacing and motion via patient-specific bone morphology had been assessed successfully in-vitro and in-vivo in pilot studies aimed at including these evaluations in traditional navigated TKA.

The aim of this study was to report the current experiences in-vivo in two patient cohorts during TKA with patellar resurfacing.

INTRODUCTION

In Total Knee Arthroplasty (TKA), the neutral overall limb alignment (NOLA), i.e. the mechanical alignment of the lower limb within 0°±3°, is targeted for achieving good clinical/functional results. The kinematic overall limb alignment (KOLA), which uses the axis through the centres of the femur posterior condyles modelled as cylinders, represents a novel approach for achieving better soft tissue balance.

Patient-specific instrumentation (PSI) is nowadays offered as an effective technology in TKA to obtain better lower limb alignments than those via conventional guides (CON). Although relevant results are still inconsistent, the benefits claimed include shorter operative time, reduced surgical instrumentation, and accurate preoperative planning.

The aim of this study was to report the preliminary clinical and radiological results of TKA patients operated via NOLA-PSI and KOLA-PSI. Comparisons between them and with the results obtained via NOLA-CON were performed.

INTRODUCTION

Despite a large percentage of total knee arthroplasty failures occurs for disorders at the patello-femoral joint (PFJ), current navigation systems report tibio-femoral (TFJ) kinematics only, and do not track the patella. Despite this tracking is made difficult by the small bone and by its full eversion during surgery, a new such technique has been developed, which includes a new tracker, new corresponding surgical instrumentation also for patellar resurfacing, and all relevant software. The aim of this study is to report an early experience in patients of these measurements, i.e. TFJ and PFJ kinematics.

During total knee replacement (TKR), knee surgical navigation systems (KSNS) report in real time relative motion data between the tibia and the femur from the patient under anaesthesia, in order to identify best possible locations for the corresponding prosthesis components. These systems are meant to support the surgeon for achieving the best possible replication of natural knee motion, compatible with the prosthesis design and the joint status, in the hope that this kinematics under passive condition will be then the same during the daily living activities of the patient. Particularly, by means of KSNS, knee kinematics is tracked in the original arthritic joint at the beginning of the operation, intra-operatively after adjustments of bone cuts and trial components implantation, and after final components implantation and cementation. Rarely the extent to which the kinematics in the latter condition is then replicated during activity is analysed. As for the assessment of the active motion performance, the most accurate technique for the in-vivo measurements of replaced joint kinematics is three-dimensional video-fluoroscopy. This allows joint motion tracking under typical movements and loads of daily living. The general aim of this study is assessing the capability of the current KSNS to predict replaced joint motion after TKR. Particularly, the specific objective is to compare, for a number of patients implanted with two different TKR prosthesis component designs, knee kinematics obtained intra-operatively after final component implantation measured by means of KSNS with that assessed post-operatively at the follow-up by means of three-dimensional video-fluoroscopy.

Thirty-one patients affected by primary gonarthrosis were implanted with a fixed bearing posterior-stabilized TKR design, either the Journey® (JOU; Smith&Nephew, London, UK) or the NRG® (Stryker®-Orthopaedics, Mahwah, NJ-USA). All implantations were performed by means of a KSNS (Stryker®-Leibinger, Freiburg, Germany), utilised to track and store joint kinematics intra-operatively immediately after final component implantation (INTRA-OP). Six months after TKR, the patients were followed for clinical assessment and three-dimensional video fluoroscopy (POST-OP). Fifteen of these patients, 8 with the JOU and 7 with the NRG, gave informed consent and these were analyzed. At surgery (INTRA-OP), a spatial tracker of the navigation system was attached through two bi-cortical 3 mm thick Kirschner wires to the distal femur and another to the proximal tibia. The conventional navigation procedure recommended in the system manual was performed to calculate the preoperative deformity including the preoperative lower limb alignment, to perform the femoral and tibial bone cuts, and to measure the final lower limb alignment. All these assessment were calculated with respect to the initial anatomical survey, the latter being based on calibrations of anatomical landmarks by an instrumented pointer. Patients were then analysed (POST-OP) by three-dimensional video-fluoroscopy (digital remote-controlled diagnostic Alpha90SX16; CAT Medical System, Rome-Italy) at 10 frames per second during chair rising-sitting, stair climbing, and step up-down. A technique based on CAD-model shape matching was utilised for obtaining three-dimensional pose of the prosthesis components. Between the two techniques, the kinematics variables analysed for the comparison were the three components of the joint rotation (being the relative motion between the tibial and femoral components represented using a standard joint convention, the translation of the line through the medial and lateral contact points (being these points assumed to be where the minimum distance between the femoral condyles and the tibial baseplate is observed) on the tibial baseplate and the corresponding pivot point, and the location of the instantaneous helical axes with the corresponding mean helical axis and pivot point.

In all patients and in both conditions, physiological ranges of flexion (from −5° to 120°), and ab-adduction (±5°) were observed. Internal-external rotation patterns are different between the two prostheses, with a more central pivoting in NRG and medial pivoting in JOU, as expected by the design. Restoration of knee joint normal kinematics was demonstrated also by the coupling of the internal rotation with flexion, as well as by the roll-back and screw-home mechanisms, observed somehow both in INTRA- and POST-OP measurements. Location of the mean helical axis and pivot point, both from the contact lines and helical axes, were very consistent over time, i.e. after six months from intervention and in fully different conditions. Only one JOU and one NRG patient had the pivot point location POST-OP different from that INTRA-OP, despite cases of paradoxical translation.

In all TKR knees analysed, a good restoration of normal joint motion was observed, both during operation and at the follow-up. This supports the general efficacy of the surgery and of both prosthesis designs. Particularly, the results here reported show a good consistency of the measurements over time, no matter these were taken in very different joint conditions and by means of very different techniques. Intra-operative kinematics therefore does matter, and must be taken into careful consideration for the implantation of the prosthesis components. Joint kinematics should be tracked accurately during TKR surgery, and for this purpose KSNS seem to offer a very good support. These systems not only supports in real time the best possible alignment of the prosthesis components, but also make a reliable prediction of the motion performance of the replaced joint. Additional analyses will be necessary to support this with a statistical power, and to identify the most predicting parameters among the many kinematics variables here analysed preliminarily.

During total knee replacement (TKR), surgical navigation systems (SNS) allow accurate prosthesis component implantation by tracking the tibio-femoral joint (TFJ) kinematics in the original articulation at the beginning of the operation, after relevant trial components implantation, and, ultimately, after final component implantation and cementation. It is known that TKR also alters normal patello-femoral joint (PFJ) kinematics resulting frequently in PFJ disorders and TKR failure. More importantly, patellar tracking in case of resurfacing is further affected by patellar bone preparation and relevant component positioning. The traditional technique used to perform patellar resurfacing, even in navigated TKR, is based only on visual inspection of the patellar articular aspect for clamping patellar cutting jig and on a simple calliper to check for patellar thickness before and after bone cut, and, thus, without any computer assistance. Even though the inclusion in in-vivo navigated TKR of a procedure for supporting also patellar resurfacing based on patient-specific bone morphology seems fundamental, this have been completely disregarded till now, whose efficacy being assessed only in-vitro. This procedure has been developed, together with relevant software and surgical instrumentation, as an extension of current SNS, i.e. TKR is navigated, at the same time measuring the effects of every surgical action on PFJ kinematics. The aim of this study was to report on the first in-vivo experiences during TKR with patellar resurfacing.

Four patients affected by primary gonarthrosis were implanted with a fixed bearing posterior-stabilised prosthesis (NRG, Stryker®-Orthopaedics, Mahwah, NJ-USA) with patellar resurfacing. All TKR were performed by means of two SNS (Stryker®-Leibinger, Freiburg, Germany) with the standard femoral/tibial trackers, the pointer, and a specially-designed patellar tracker. The novel procedure for patellar tracking was approved by the local ethical committee; the patients gave informed consent prior the surgery. This procedure implies the use of a second system, i.e. the patellar SNS (PSNS), with dedicated software for supporting patellar resurfacing and relative data processing/storing, in addition to the traditional knee SNS (KSNS). TFJ anatomical survey and kinematics data are shared between the two. Before surgery, both systems were initialised and the patellar tracker was assembled with a sterile procedure by shaping a metal grid mounted with three markers to be tracked by PSNS only. The additional patellar-resection-plane and patellar-cut-verification probes were instrumented with a standard tracker and a relevant reference frame was defined on these by digitisation with PSNS. Afterwards, the procedures for standard navigation were performed to calculate preoperative joint deformities and TFJ kinematics. The anatomical survey was performed also with PSNS, with relevant patellar anatomical reference frame definition and PFJ kinematics assessment according to a recent proposal. Standard procedures for femoral and tibial component implantation, and TFJ kinematics assessment were then performed by using relevant trial components. Afterwards, the procedure for patellar resection begun. Once the surgeon had arranged and fixed the patellar cutting jig at the desired position, the patellar-resection-plane probe was inserted into the slot for the saw blade. With this in place, the PSNS captured tracker data to calculate the planned level of patellar bone cut and the patellar cut orientation. Then the cut was executed, and the accuracy of this actual bone cut was assessed by means of the patellar-cut-verification probe. The trial patellar component was positioned, and, with all three trial components in place, TFJ and PFJ kinematics were assessed. Possible adjustments in component positioning could still be performed, until both kinematics were satisfactory. Finally, final components were implanted and cemented, and final TFJ and PFJ kinematics were acquired. A sterile calliper and pre- and post-implantation lower limb X-rays were used to check for the patellar thickness and final lower limb alignment. The novel surgical technique was performed successfully in all four cases without complication, resulting in 30 min longer TKR. The final lower limb alignment was within 0.5°, the resurfaced patella was 0.4±1.3 mm thinner than in the native, the patellar cut was 1.5°±3.0° laterally tilted. PFJ kinematics was taken within the reference normality. The patella implantation parameters were confirmed also by X-ray inspection; discrepancies in thickness up to 5 mm were observed between SNS- and calliper-based measurements.

At the present experimental phase, a second separate PSNS was utilised not to affect the standard navigated TKR. The results reported support relevance, feasibility and efficacy of patellar tracking and PFJ kinematics assessment in in-vivo navigated TKR. The encouraging in-vivo results may lay ground for the design of a future clinical patella navigation system the surgeon could use to perform a more comprehensive assessment of the original whole knee anatomy and kinematics, i.e. including also PFJ. Patellar bone preparation would be supported for suitable patellar component positioning in case of resurfacing but, conceptually, also in not resurfacing if patellar anatomy and tracking assessment by SNS reveals no abnormality. After suitable adjustment and further tests, in the future if this procedure will be routinely applied during navigated TKR, abnormalities at both TFJ and PFJ can be corrected intra-operatively by more cautious bone cut preparation on the femur, tibia and also patella, in case of resurfacing, and by correct prosthetic component positioning.

Restoration of natural range and pattern of motion is the primary goal of joint replacement. In total ankle replacement, proper implant positioning is a major requirement to achieve good clinical results and to prevent instability, aseptic loosening, meniscal bearing premature wear and dislocation at the replaced ankle. The current operative techniques support limitedly the surgeon in achieving a best possible prosthetic component alignment and in assessing proper restoration of ligament natural tensioning, which could be well aided by computer-assisted surgical systems. Therefore the outcome of this replacement is, at present, mainly associated to surgeon's experience and visual inspection. In some of the current ankle prosthetic designs, tibial component positioning along the anterior/posterior (A/P) and medio/lateral axes is critical, particularly in those designs not with a flat articulation between the tibial and the meniscal or talar components. The general aim of this study was assessing in-vitro the effects of the A/P malpositioning of the tibial component on three-dimensional kinematics of the replaced joint and on tensioning of the calcaneofibular (CaFiL) and tibiocalcaneal (TiCaL) ligaments, during passive flexion. Particularly, the specific objective is to compare the intact ankle kinematics with that measured after prosthesis component implantation over a series of different positions of the tibial component.

Four fresh-frozen specimens from amputation were analysed before and after implantation of an original convex-tibia fully-congruent three-component design of ankle replacement (Box Ankle, Finsbury Orthopaedics, UK). Each specimen included the intact tibia, fibula and ankle joint complex, completed with entire joint capsule, ligaments, muscular structures and skin. The subtalar joint was fixed with a pin protruding from the calcaneus for isolating tibiotalar joint motion. A rig was used to move the ankle joint complex along its full range of flexion while applying minimum load, i.e. passive motion. In these conditions, motion at the ankle was constrained only by the articular surfaces and the ligaments. A stereofotogrammetric system for surgical navigation (Stryker-Leibinger, Freiburg, Germany) was used to track the movement of the talus/calcaneus and tibial segments, by using trackers instrumented with five active markers. Anatomical based kinematics was obtained after digitization by an instrumented pointer of a number of anatomical landmarks and by a standard joint convention. The central point of the attachment areas of CaFiL e TiCaL was also digitised. Passive motion and ankle joint neutral position were acquired, and the standard operative technique was performed to prepare the bones for prosthesis component implantation. The final component for the talus was implanted, the tibial component was initially positioned well in front of the nominal right (NR) position, the meniscal bearing was instrumented with an additional special tracker, and passive motion was collected again in passive flexion. Data collection was repeated for progressively more posterior locations for the tibial component, for a total of six different locations along the tibial A/P axis: three anterior (PA), the NR, and two more posterior (PP), approximately 3 to 5 mm far apart each. The following three-dimensional kinematics variables were analyzed: the three anatomical components of the ankle joint (talus-to-tibial) rotation (dorsi/plantar flexion, prono/supination and internal/external rotation respectively in the sagittal, frontal and transverse planes), the meniscal bearing pose with respect to the talar and tibial components, the ‘ligament effective length fraction’ as the ratio between the instantaneous distance between the ligament attachment points and the corresponding maximum distance, and the instantaneous and mean helical axes in the tibial anatomical reference frame.

In all specimens and in all conditions, physiological ranges of flexion, prono/supination and internal/external rotation were observed at the ankle joint. A good restoration of motion was observed at the replaced joint, demonstrated also by the coupling between axial rotation and flexion and the physiological location of the mean helical axis, in all specimens and in most of the component positions. Larger plantar- and smaller dorsi-flexion were observed when the tibial component was positioned more anteriorly than NR, and the opposite occurred for more posterior positions. In regards to the meniscal bearing, rotations were small and followed approximately the same patterns of the ankle rotations, accounted for the full conformity of the articulating surfaces. Translations in A/P were larger than in other directions, the bearing moving backward in plantarflexion and forward in dorsiflexion with respect to both components. It was observed that the closer to NR the position of the tibial component is, the larger this A/P motion is, accounted mainly to the associated larger range of flexion. The change of CaFiL and TiCaL effective length fraction over the flexion arc was found smaller than 0.1 in three specimens, smaller than 0.2 in the fourth, larger both in more anterior and more posterior locations of the tibial component. The simulated malpositioning did not affect much position and orientation of the mean helical axis in both the transversal and frontal planes.

The experimental protocol and measurements were appropriate to achieve the proposed goals. All kinematics variables support the conclusion that the ankle replaced with this original prosthesis behaves as predicted by the relevant computer models, i.e. physiological joint motion and ligament tension is experienced resulting in a considerable A/P motion of the meniscal bearing. These observations are particularly true in the NR postion for the prosthesis, but are somehow correct also in most of the tibial malpositions analysed, in particular those on the back.

Computer-assisted techniques in total knee replacement (TKR) have been introduced to improve bone cuts execution and relevant prosthesis components positioning. Although these have resulted in good surgical outcomes when compared to the conventional TKR technique, the surgical time increase and the use of additional invasive devices remain still critical. In order to cope with these issues, a new technology in TKR has been introduced also for positioning prosthetic components according to the natural lower-limb alignment. This technique is based on custom-fit cutting block derived from patient-specific lower-limb scan acquisition. The purpose of this study is to assess the accuracy of the custom-fit technology by means of a knee surgical navigation system, here used only as measurement system, and post-operative radiographic evaluations. Particularly, the performances of two different custom-fit cutting blocks realized from as many scan acquisitions have been here reported.

Thirty patients affected by primary knee osteoarthritis were enrolled in this study. Fifteen patients were implanted with GMK® (Medacta-International, Castel San Pietro, CH) and as many patients with Journey® (Smith&Nephew, London, UK). Both TKR designs were implanted by using custom-fit blocks for bone cut executions provided by the same TKR manufacturers according to a pre-operative web planning approved by the surgeon. Particularly, the cutting block for the former design was built from CT scan acquisition of the hip, knee and ankle, whereas that for the latter design from MRI scans acquisition of the knee and X-ray lower-limb overview. A knee surgical navigation system (Stryker®-Leibinger, Freiburg, Germany) was used for recording intra-operative alignment of bone cuts as performed by means of the custom-fit cutting blocks and relevant component positioning. Prosthetic components alignments were also assessed post-operatively on X-ray images according to a shape-matching technique. The accuracy of the custom-fit blocks was evaluated through the comparison between pre-operative planning, and intra/post-operative data. Discrepancies above 3° and millimeters were considered as outliers.

Within the patient cohort, nine cases were fully analyzed at the moment and here reported. Over them and except for one case, the discrepancy between pre-operative planned femoral/tibial resection level on the frontal plane and the corresponding measured intra-operatively was within 3 mm, being 5 mm in the worse case. Two outliers were observed for the corresponding femoral/tibial cut rotational alignment. Particularly, in one patient, the discrepancy in femoral cut alignment was of 8° in flexion and 6° in external rotation; in another patient this was of 4° in extension and 4° in external rotation in the femoral and tibial cut alignment, respectively. Post-operative radiographs evaluations for the final prosthetic components revealed that femoral/tibial alignment were within 3° in all cases, except for those patients that were already outliers.

These preliminary results reveal the efficacy of the custom-fit cutting block for TKR. These were generally fitted properly and final prosthetic components were accurately placed, although some discrepancies were observed. This new technology seems to be a valid alternative to conventional and computer-assisted techniques. More consistent conclusions can be deduced after final evaluation of all patients.