**1. Introduction**

It is essential to understand the anatomy and biomechanics of the knee joint before performing a sub-total or total meniscectomy due to the possible catastrophic consequences at a long-term follow-up. Moreover, the medial and the lateral compartment of the knee have different kinematic properties and the clinician must take into account the different degree of mobility, bony structure, and load distribution between these two compartments. Biomechanical studies have demonstrated the essential role of the menisci on load transfer, in that a total meniscectomy can increase the contact area by 33 to 50 percent in a fully extended knee [1].

Walker et al. demonstrated that the lateral compartment is much more dependent on meniscal function than the medial one. The lateral meniscus carries a higher percentage of load transfer than the medial meniscus. This is due to a higher load being transferred directly by the exposed cartilage surface of the medial compartment [1]. The different bony morphology of the tibiofemoral compartments also play a part in this. In the sagittal plane, the medial convexity of the femoral condyle and the concavity of the tibial plateau provide a degree of congruity, even after a meniscectomy. While, on the lateral side, both the convexity of the femoral condyle and the lateral tibial plateau make this compartment much more prone to an increase in peak contact pressures after meniscectomy [2].

Clinically, the differences between the medial and lateral meniscus have been confirmed by worse results reported after lateral meniscectomy compared to medial meniscectomy at a long-term follow-up [3, 4]. These findings are even more stark if the meniscectomy is performed during adolescence: in a prospective 30 years of follow-up study, about 80% of patients after medial meniscectomy maintained good or excellent clinical results; in comparison to less than 50% for lateral meniscectomy [5].

The causal relationship of knee arthritis with meniscectomy led to the investigation of meniscal allograft transplantation for the post-meniscectomy patient experiencing pain and demonstrating arthritic changes [6]. Basic science studies have shown that although meniscal allografts cannot fully replicate the function of the native meniscus, the grafts are able to significantly improve joint contact area and decrease contact pressures [7, 8]. Also, early clinical series demonstrated isolated meniscal allograft transplantation to be a feasible procedure [9, 10]. However, the initial studies had variable outcomes due to significant differences in indications, surgical techniques, and tissue processing methods.

As experience was gained, the variables became more defined and the results improved. Numerous short and midterm studies have shown that meniscal allografts are able to provide pain relief and increase function with high rates of graft survivorship [11, 12]. More recently, a long-term study has been published demonstrating >50% graft survival at 20 years [13].

However, if a subtotal meniscectomy has previously been performed, a meniscal scaffold may be a more appropriate procedure, despite the relative lack of relevant articles with extended follow-up. There are two different scaffold types available on the market: the collagen meniscus implant (CMI) derived from a bovine collagen and the Actifit, a polyurethane scaffold [14, 15]. 3D printed scaffolds have been recently proposed as an experimental treatment and only a few case reports are available, while CMI and Actifit have been widely studied.
