The reliable production of _in vitro_ chondrocytes that faithfully recapitulate _in vivo_ development would be of great benefit for orthopaedic disease modelling and regenerative therapy(1,2). Current efforts are limited by off-target differentiation, resulting in a heterogeneous product, and by the lack of comparison to human tissue, which precludes detailed evaluation of _in vitro_ cells(3,4).

We performed single-cell RNA-sequencing of long bones dissected from first-trimester fetal limbs to form a detailed ‘atlas’ of endochondral ossification. Through 100-gene in-situ sequencing, we placed each sequenced cell type into its anatomical context to spatially resolve the process of endochondral ossification. We then used this atlas to perform deconvolution on a series of previously published bulk transcriptomes generated from _in vitro_ chondrogenesis protocols to evaluate their ability to accurately produce chondrocytes.

We then applied single-nuclear RNA-sequencing to cells from the best performing protocol collected at multiple time points to allow direct comparison between the differentiation of _in vitro_ and _in vivo_ cells.

We captured 275,000 single fetal cells, profiling the development of chondrocytes from multipotent mesenchymal progenitors to hypertrophic cells at full transcriptomic breadth. Using this atlas as the ground truth for evaluating _in vitro_ cells, we found substantial variability in cell states produced by each protocol, with many showing little similarity to _in vivo_ cells, and all exhibiting off-target differentiation.

Trajectory alignment between _in vivo_ and _in vitro_ single-cell data revealed key differences in gene expression dynamics between _in vitro_ and _in vivo cells,_ with several osteoblastic transcription factors erroneously unregulated _in vitro,_ including _FOXO1._

Using this information, we inhibited _FOXO1_ in culture to successfully increase chondrocyte yield _in vitro._

This study presents a new framework for evaluating tissue engineering protocols, using single-cell data to drive improvement and bring the prospect of true engineered cartilage closer to reality.

Background

Intracapsular neck of femur fractures are one of the most common injuries seen in Orthopaedics. When the fracture is amenable to internal fixation there are 2 main treatment options, namely multiple cannulated hip screws (MCS) and 2-hole sliding hip screws (SHS). In this retrospective study we examine the outcomes associated with these two methods of internal fixation. At present there is little consensus regarding which treatment should be used

Statement of Purpose:

The wear rate of Ultra High Molecular Weight Polyethylene (UHMWPE) in joint replacements has been correlated to both contact area and contact stress in the literature, [1], [2]. In both publications and our experiment, UHMWPE articulated with a polished surface of cobalt-chromium alloy was evaluated using a Pin-On-Disk (POD) apparatus (AMTI) implementing bi-directional movement.

In publication [1], volumetric wear was independent of normal load and dependent upon increasing contact area. The results demonstrated that increasing contact stress decreased wear rates twofold. In publication [2], at maximum cross-shear, wear was proportional to nominal contact area and wear factors normalized to area are more appropriate than load based wear factors. In both studies, the contact surface areas of the POD pins were reduced by decreasing the diameters of the POD Pins.

In our experiment, the contact area was dependent on textured POD Pin 390 (T390) which had low wear [3]. T390 reduced the normal POD contact area from 71 mm² to 8.26 mm². Hydroxylapatite (HA) particles were introduced to the serum to simulate third body wear debris. We hypothesized that the normal POD Pins would have greater wear rates than the textured POD Pins. A measurement of 0.14 mg HA particles per 250 mL of serum was used for each test 0.33 million cycles.