A cage assembly for implanting between and securing together two portions of bone (10, 14) in a tibial tuberosity advancement, the cage assembly comprising: an elongated cage body having a first end, a second end, opposing sides (28, 30), a top and a bottom, the cage body for placement between said two portions of bone such that each side contacts one of said portions of bone so as to maintain a space between the portions of bone; and a securing member configured to interlock with the cage body and for securing to said portions of bone; wherein the cage body comprises a box portion having a top wall, a bottom wall, a first end wall at the first end of the cage body and a second end wall towards the second end of the cage body; characterised in that: each of the top wall and bottom wall of the box portion is a single continuous rectangle; the cage body is made of a bio-absorbable polymer that will be absorbed into an animal body over time when implanted within the animal body; and the box portion comprises a support member extending from the corner formed between the bottom wall and the second end wall to the corner formed between the top wall and the first end wall; and/or wherein the box portion comprises a support member extending from the corner formed between the bottom wall and the first end wall to the corner formed between the top wall and the second end wall. an elongated cage body having a first end, a second end, opposing sides (28, 30), a top and a bottom, the cage body for placement between said two portions of bone such that each side contacts one of said portions of bone so as to maintain a space between the portions of bone; and a securing member configured to interlock with the cage body and for securing to said portions of bone; wherein the cage body comprises a box portion having a top wall, a bottom wall, a first end wall at the first end of the cage body and a second end wall towards the second end of the cage body; characterised in that: each of the top wall and bottom wall of the box portion is a single continuous rectangle; the cage body is made of a bio-absorbable polymer that will be absorbed into an animal body over time when implanted within the animal body; and the box portion comprises a support member extending from the corner formed between the bottom wall and the second end wall to the corner formed between the top wall and the first end wall; and/or wherein the box portion comprises a support member extending from the corner formed between the bottom wall and the first end wall to the corner formed between the top wall and the second end wall. each of the top wall and bottom wall of the box portion is a single continuous rectangle; the cage body is made of a bio-absorbable polymer that will be absorbed into an animal body over time when implanted within the animal body; and the box portion comprises a support member extending from the corner formed between the bottom wall and the second end wall to the corner formed between the top wall and the first end wall; and/or wherein the box portion comprises a support member extending from the corner formed between the bottom wall and the first end wall to the corner formed between the top wall and the second end wall.