Aims

Manual impaction, with a mallet and introducer, remains the standard method of installing cementless acetabular cups during total hip arthroplasty (THA). This study aims to quantify the accuracy and precision of manual impaction strikes during the seating of an acetabular component. This understanding aims to help improve impaction surgical techniques and inform the development of future technologies.

Abstract

OSSTEC is a pre-spin-out venture at Imperial College London seeking industry feedback on our orthopaedic implants which maintain bone quality in the long term. Existing orthopaedic implants provide successful treatment for knee osteoarthritis, however, they cause loss of bone quality over time, leading to more dangerous and expensive revision surgeries and high implant failure rates in young patients.

OSSTEC tibial implants stimulate healthy bone growth allowing simple primary revision surgery which will provide value for all stakeholders. This could allow existing orthopaedics manufacturers to capture high growth in existing and emerging markets while offering hospitals and surgeons a safer revision treatment for patients and a 35% annual saving on lifetime costs. For patients, our implant technology could mean additional years of quality life by revising patients to a primary TKA before full revision surgery.

Our implants use patent-filed additive manufacturing technology to restore a healthy mechanical environment in the proximal tibia; stimulating long term bone growth. Proven benefits of this technology include increased bone formation and osseointegration, shown in an animal model, and restoration of native load transfer, shown in a human cadaveric model.

This technology could help capture the large annual growth (24%) currently seen in the cementless knee reconstruction market, worth $1.2B. Furthermore, analysis suggests an additional market of currently untreated younger patients exists, worth £0.8B and growing by 18% annually. Making revision surgery and therefore treatment of younger patients easier would enable access to this market. We aim to offer improved patient treatment via B2B sales of implants to existing orthopaedic manufacturer partners, who would then provide them with instrumentation to hospitals and surgeons.

Existing implant materials provide good options for patient treatments, however OSSTEC's porous titanium structures offer unique competitive advantages; combining options for modular design, cementless fixation, initial bone fixation and crucially long term bone maintenance.

Speaking to surgeons across global markets shows that many surgeons are keen to pursue bone preserving surgeries and the use of porous implants. Furthermore, there is a growing demand to treat young patients (with 25% growth in patients younger than 65 over the past 10 years) and to use cementless knee treatments, where patient volume has doubled in the past 4 years and is following trends in hip treatments.

Our team includes engineers and consultant surgeons who have experience developing multiple orthopaedic implants which have treated over 200,000 patients. To date we have raised £175,000 for the research and development of these implants and we hope to gain insight from industry professionals before further development towards our aim to begin trials for regulatory approval in 2026.

OSSTEC implants provide a way to stimulate bone growth after surgery to reduce revision risk. We hope this could allow orthopaedic manufactures to explore high growth markets while meaning surgeons can treat younger patients in a cost effective way and add quality years to patients' lives.

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Surgical management of cam-type femoroacetabular impingement (FAI) aims to preserve the native hip, restore joint function, and delay the onset of osteoarthritis. However, it is unclear how surgery affects joint mechanics and hip joint stability. The aim was to examine the contributions of each surgical stage (i.e., intact cam hip, capsulotomy, cam resection, capsular repair) towards hip joint centre of rotation and microinstability.

Twelve fresh, frozen cadaveric hips (n = 12 males, age = 44 ± 9 years, BMI = 23 ± 3 kg/m2) were skeletonized to the capsule and included in this study. All hips indicated cam morphology on CT data (axial α = 63 ± 6°, radial α = 74 ± 4°) and were mounted onto a six-DOF industrial robot (TX90, Stäubli). The robot positioned each hip in four sagittal angles: 1) Extension, 2) Neutral 0°, 3) Flexion 30°, and 4) Flexion 90°, and performed internal and external hip rotations until a 5-Nm torque was reached in each direction, while recording the hip joint centre's neutral path of translation. After the (i) intact hip was tested, each hip underwent a series of surgical stages and was retested after each stage: (ii) T-capsulotomy (incised lateral iliofemoral capsular ligament), (iii) cam resection (removed morphology), and (iv) capsular repair (sutured portal incisions). Eccentricity of the hip joint centre was quantified by the microinstability index (MI = difference in rotational foci / femoral head radius). Repeated measures ANOVA and post-hoc paired t-tests compared the within-subject differences in hip joint centre and microinstability index, between the testing stages (CI = 95%, SPSS v.24, IBM).

At the Extension and Neutral positions, the hip joint centre rotated concentrically after each surgical stage. At Flexion 30°, the hip joint centre shifted inferolaterally during external rotation after capsulotomy (p = 0.009), while at Flexion 90°, the hip joint centre further shifted inferolaterally during external rotation (p = 0.005) and slightly medially during internal rotation after cam resection, compared to the intact stages. Consequently, microinstability increased after the capsulotomy at Flexion 30° (MI = +0.05, p = 0.003) and substantially after cam resection at Flexion 90° (MI = +0.07, p = 0.007). Capsular repair was able to slightly restrain the rotational centre and decrease microinstability at the Flexion 30° and 90° positions (MI = −0.03 and −0.04, respectively).

Hip microinstability occurred at higher amplitudes of flexion, with the cam resection providing more intracapsular volume and further lateralizing the hip joint during external rotation. Removing the cam deformity and impingement with the chondrolabral junction also medialized the hip during internal rotation, which can restore more favourable joint loading mechanics and stability. These findings support the pathomechanics of cam FAI and suggest that iatrogenic microinstability may be due to excessive motions, prior to post-operative restoration of static (capsular) and dynamic (muscle) stability. In efforts to limit microinstability, proper nonsurgical management and rehabilitation are essential, while activities that involve larger amplitudes of hip flexion and external rotation should be avoided immediately after surgery.

Introduction

Integrating additively manufactured structures, such as porous lattices into implants has numerous potential benefits, such as custom mechanical properties, porosity for osseointegration/fluid flow as well as improved fixation features.

Component anisotropic stiffness can be controlled through varying density and lattice orientation. This is useful due to the influence of load on bone remodelling. Matching implant and bone anisotropy/stiffness may help reduce problems such as stress shielding and prevent implant loosening. It is therefore beneficial to be able to design AM parts with a desired anisotropic stiffness.

In this study we present a method that predicts the anisotropic stiffness of an additively manufactured lattice structure from its CAD data, and validate this model with experimental testing. The model predicts anisotropic stiffness in terms of density (ρ), fabric (M) and fabric eigen values (m) and is matched to stiffness data of the structure in 3 principal directions, based on an orthotropic assumption. This model was described in terms of 10 constants and had the form shown in Equation 1.

Eq.1 S = ∑ i , j = 1         i , j = 3 λ ( i , j ) ρ k m ( i ) 1 ( i ) m ( j ) 1 ( i ) | M i M j ' | 2

Background

Cementless acetabular cups rely on press-fit fixation for initial stability; an essential pre-requisite to implant longevity. Impaction is used to seat an oversized implant in a pre-prepared bone cavity, generating bone strain, and ‘grip’ on the implant. In certain cases (such as during revision) initial fixation is more difficult to obtain due to poorer bone quality. This increases the chance of loosening and instability. No current study evaluates how a surgeon's impaction technique (mallet mass, mallet velocity and number of strikes) may be used to maximise cup fixation and seating.

Incidence of intraoperative fracture during cementless Total Hip Arthroplasty (THA) is increasing. This is attributed to factors such as an increase in revision procedures and the favour of cementless fixation. Intraoperative fractures often occur during the seating of cementless components. A surgical mallet and introducer are used to generate the large impaction forces necessary to seat the component, sometimes leading to excessive hoop strain in the bone. The mechanisms of bone strain during impaction are complex and occur over very short timeframes. For this reason experimental and simulation models often focus on strain shortly after the implant is introduced, or seat it quasi-statically. This may not produce a realistic representation of the magnitude of strain in the bone and dangerously under-represent fracture risk.

This in-vitro study seeks to determine whether strain induced during impaction is similar both during the strike (dynamic strain) and shortly after the strike has occurred (post-strike strain). It is also asked whether post-strike strain is a reliable predictor of dynamic strain.

A custom drop tower was used to seat acetabular components in 45 Sawbones models (SKU: 1522–02, Malmo, Sweden), CNC milled to represent the acetabular cavity. Ten strikes were used to seat each cup. 3 strike velocities (1.5 m/s, 2.75 m/s, 4 m/s) and 3 impact masses (600 g, 1.2 kg, 1.8 kg) were chosen to represent 9 different surgical scenarios. Two strain gages per Sawbone were mounted on the surface of the block, 2 mm from the rim of the cavity. Strain data was acquired at 50 khz. Each strain trace was then analysed to determine the peak dynamic strain during mallet strike and the static strain post-strike.

A typical strain pattern was observed during seating. An initial pre-strike strain is followed by a larger dynamic peak as the implant is progressed into the bone cavity. Strain subsequently settles at a lower (tensile) value than peak dynamic post-strike, but higher than pre-strike strain. Over the 450 strikes conducted dynamic strain was on average 3.39 times larger than post-strike strain. A statistically significant linear relationship was observed between the magnitude of post-strike and dynamic strain (adjusted R²=0.391, p<0.005). This indicates that, for a known scenario, post-strike strain can be used as an indicator for dynamic peak strain. However when only the maximum dynamic and post-strike strains were taken from across the 10 strikes used to seat the implant, the relationship between the two strains was not significant (R²=0.300, p=0.73). This may be due to the fact that the two maximums did not often occur on the same strike. On average, max dynamic strain occurred 1.7 strikes after max post-strike strain.

We conclude that peak dynamic strain is much larger than the strain immediately post-strike in a synthetic bone model. It is shown that post-strike strain is not a good predictor of dynamic strain when the max strain during any strike to seat the component is considered, or variables (such as mallet mass or velocity) are changed. It is important to consider dynamic strain in bone as well as post-strike strain in experimental or simulated bone models to ensure the most reliable prediction of fracture.

Initial stability of cementless components in bone is essential for longevity of Total Hip Replacements. Fixation is provided by press-fit: seating an implant in an under-reamed bone cavity with mallet strikes (impaction). Excessive impaction energy has been shown to increase the risk of periprosthetic fracture of bone. However, if implants are not adequately seated they may lack the stability required for bone ingrowth. Ideal fixation would maximise implant stability but would minimise peak strain in bone, reducing the risk of fracture.

This in-vitro study examines the influence of impaction energy and number of seating strikes upon implant push-out force (indicating stability) and peak dynamic strain in bone substitute (indicating likelihood of fracture). The ratio of these factors is given as an indicator of successful impaction strategy.

A custom drop tower with simulated hip compliance was used to seat acetabular cups in 30 Sawbone blocks with CNC milled acetabular cavities. 3 impaction energies were selected; low (0.7j), medium (4.5j) and high (14.4j), representing the wide range of values measured during surgery. Each Sawbone was instrumented with strain gauges, secured on the block surface close to the acetabular cavity (Figure 1). Strain gauge data was acquired at 50 khz with peak tensile strain recorded for each strike. An optical tracker was used to determine the polar gap between the cup and Sawbone cavity during seating. Initially 10 strikes were used to seat each cup. Tracking data were then used to determine at which strike the cups progressed less than 10% of the final polar gap. This value was taken as number of strikes to complete seating. Tests were repeated with fresh Sawbone, striking each cup the number of times required to seat. Following each seating peak push-out forces of the cups were recorded using a compression testing machine.

10, 5 and 2 strikes were required to seat the acetabular cups for the low, medium and high energies respectively. It was found that strain in the Sawbone peaked around the number of strikes to complete seating and subsequently decreased. This trend was particularly pronounced in the high energy group. An increase in Sawbone strain during seating was observed with increasing energy (270 ± 29 µε [SD], 519 ± 91 µε and 585 ± 183 µε at low, medium and high energies respectively). The highest push-out force was achieved at medium strike energy (261 ± 46N). The ratio between push-out and strain was highest for medium strike energy (0.50 ± 0.095 N/µε). Push-out force was similar after 5 and 10 strikes for the medium energy strike. However push-out recorded at ten strikes for the high energy group was significantly lower than for 2 strikes (<40 ± 19 N, p<0.05).

These results indicate that a medium strike energy with an appropriate number of seating strikes maximizes initial implant stability for a given peak bone strain. It is also shown that impaction with an excessive strike energy may greatly reduce fixation strength while inducing a very high peak dynamic strain in the bone. Surgeons should take care to avoid an excessive number of impaction strikes at high energy.

For any figures or tables, please contact the authors directly.

The hip's capsular ligaments (CL) passively restrain extreme range of motion (ROM) by wrapping around the native femoral head/neck, and protect against impingement and instability. We compared how CL function was affected by device (hip resurfacing arthroplasty, HRA; dual mobility total hip arthroplasty, DM-THA; and conventional THA, C-THA), and surgical approach (anterior and posterior), with and without CL surgical-repair. We hypothesized that CL function would only be preserved when native head-size (HRA/DM-THA) was restored.

CL function was quantified on sixteen cadaveric hips, by measuring ROM by internally (IR) and externally rotating (ER) the hip in six functional positions, ranging from full extension with abduction to full flexion with adduction (squatting). Native ROM was compared to ROM after posterior capsulotomy (right hips) or anterior capsulotomy (left hips), and HRA, and C-THA and DM-THA, before and after CL repair.

Independent of approach, ROM increased most following C-THA (max 62°), then DM-THA (max 40°), then HRA (max 19°), indicating later CL engagement and reduced biomechanical function with smaller head-size. Dislocations also occurred in squatting after C-THA and DM-THA. CL-repair following HRA restored ROM to the native hip (max 8°). CL-repair following DM-THA reduced ROM hypermobility in flexed positions only and prevented dislocation (max 36°). CL-repair following C-THA did not reduce ROM or prevent dislocation.

For HRA and repair, native anatomy was preserved and ligament function was restored. For DM-THA with repair, ligament function depended on the movement of the mobile-bearing, with increased ROM in positions when ligaments could not wrap around head/neck. For C-THA, the reduced head-size resulted in inferior capsular mechanics in all positions as the ligaments remained slack, irrespective of repair.

Choosing devices with anatomic head-sizes (HRA/DM-THA) with capsular repair may have greater effect than surgical approach to protect against instability in the early postoperative period.

We aimed to demonstrate the clinical safety of a novel anatomic cementless ceramic hip resurfacing device. Concerns around the safety of metal on metal arthroplasty have made resurfacing less attractive, while long term function continues to make the concept appealing. Biolox Delta ceramic is now used in more than 50% of all hip arthroplasties, suggesting that it's safety profile is acceptable. We wondered if a combination of these concepts might work?

The preclinical testing of anatomic hip resurfacing device developed by our group was presented last year. A twenty patient safety study was designed. Patients had to be between the ages of 18 and 70. The initial size range was restricted to femoral heads between 46 and 54, representing the common sizes of hip resurfacing. The primary outcomes were clinical safety, PROMs and radiological control. Secondary outcomes include CTRSA and metal ion levels.

20 patients were recruited, aged 30–69. 7 were women and 13 were men. There were no operative adverse events in their operations undertaken between September 2017 and February 2018. One patient had a short episode of atrial fibrillation on the second postoperative day, and no other complications. At three months the median oxford hip score had risen from 27 (range 14–38) to 46 (31–48). Cobalt and chromium levels were almost undetectable at 3 months. Fixation appeared satisfactory in all patients, with no migration detected in either component. CTRSA is in process.

The initial safety of a novel cementless ceramic resurfacing device is demonstrated by this data. The 10 year, 250 case efficacy study will continue in 5 other European centres.

The hip joint capsular ligaments (CL) passively restrain extreme range of motion (ROM) by wrapping around the native femoral head, and protect against impingement, edge loading wear and dislocation. This study compared how ligament function was affected by device (hip resurfacing arthroplasty, HRA; dual mobility total hip arthroplasty, DM-THA; and conventional THA, C-THA), with and without CL repair. It was hypothesized that ligament function would only be preserved when native anatomy was preserved: with restoration of head-size (HRA or DM-THA) and repair.

Eight normal male cadaveric hips were skeletonised, retaining the hip capsule. CL function was quantified by measuring ROM by internally (IR) and externally rotating (ER) the hip in six functional positions, ranging from full extension with abduction to full flexion with adduction (squatting). Native ROM was compared to ROM after posterior capsulotomy and HRA, and C-THA and DM-THA, before and after surgical CL repair.

ROM increased most following C-THA (max 62°), then DM-THA (max 40°), then HRA (max 19°), indicating later engagement of the capsule and reduced biomechanical function with smaller head-size. Dislocations also occurred in squatting after C-THA and DM-THA. CL-repair following HRA restored ROM to the native hip (max 8°). CL-repair following DM-THA reduced ROM hypermobility in flexed positions only and prevented dislocation (max 36°). CL-repair following C-THA did not reduce ROM or prevent dislocation.

When HRA was combined with repair, native anatomy was preserved and ligament function was restored. For DM-THA with repair, ligament function depended on the movement of the mobile bearing resulting in near-native function in some positions, but increased ROM when ligaments were unable to wrap around the head/neck. Following C-THA, the reduced head-size resulted in inferior capsular mechanics in all positions as the ligaments remained slack, irrespective of repair.

Choosing devices with anatomic head-sizes (resurfacing or dual-mobility) and repairing the capsular ligaments may protect against instability in the early postoperative period.

Background

The hip joint capsular ligaments passively restrain extreme range of motion (ROM), protecting the native hip against impingement, subluxation, edge loading and dislocation. This passive protection against instability would be beneficial following total hip arthroplasty (THA), however the reduced femoral head diameter postoperatively may prevent a wrapping mechanism that is essential to capsular ligament function in the native hip. Therefore we hypothesized that, post-THA, the reduced femoral head size would prevent the capsular ligaments protective biomechanical function.

Hip resurfacing remains a safe and effective option according to registry data. Results in women were less reliable, in part owing to soft tissue impingement. Biolox Delta ceramic bearing couples are now in widespread use with very low complication rates. We set about merging these three elements to develop a novel hip resurfacing arthroplasty.

Contours of both acetabular and femoral components were generated from biometric data, adapted to the constraints of ceramic machining, to ensure that radii blended from the bearing surface avoiding any sharp boundaries. Plasma spray coating with titanium and hydroxyapatite direct onto ceramic was developed and tested using shear, tensile and taber abrasion testing. Wear testing was carried out to 5 million cycles according to the ASTM. Destructive testing was carried out in a variety of test conditions and angles.

Cadaveric testing demonstrated stability using a single use disposable instruments for both conventional and patient specific procedures. Very low dose CT enabled the entire interface to be observed as the Ceramic is radiolucent, enhancing migration analysis, which will be undertaken at 4 intervals to confirm stability. Functional scores and gait analysis will be used in the safety study.

The CE study recruitment is underway, with first in human trials starting in summer 2017. PMA submission will follow the safety study. Commercial release of the device in Europe is unlikely before 2019, and in the USA may not be until 2027. The path to novel device development in 2017 is very costly in time and money.

The hip joint capsule passively restrains extreme range of motion protecting against impingement, dislocation and possibly edge loading. These functions would be advantageous following total hip arthroplasty (THA) however the degree of capsular excision, preservation and/or repair greatly varies between surgeons/approaches. Therefore, we asked: how does THA affect capsular ligamentous biomechanics? Which factors have the biggest influence?

For this laboratory based, cadaveric model, THA was performed through the acetabular medial wall, thus preserving the entire hip capsule. A previously published testing rig was used to measure capsular function by internally and externally rotating the hip in each of five hip positions (standing, sitting, gait heel strike, and two impingement risk positions, full flexion with adduction & extension with abduction). N=8 hips were tested both before and after THA allowing for repeated measurements between the native and replaced hip.

The ROM before the capsule engaged increased following THA (p<0.05), indicating reduced biomechanical function. Internal rotation was affected more than external rotation. Increasing neck length restored the ROM more towards the native condition. Increasing head size also had a small positive effect, but less than neck length.

Following THA, the capsular ligaments were no longer able to wrap around the smaller femoral head thereby limiting their ability to restrain excessive hip movement. The anterior capsule is affected less than the posterior, and may benefit from being preserved length. A repair to the posterior capsule should compensate for the reduced THA head size in order to restore function.