The dead-space inside one channel of the A-Tap is 0.25ml

There are several challenges to the current intra-articular injection practice:

It is challenging to keep the needle stable
Painful for patient
Increases the risk of infection
20-30% of injections miss the joint

Arthrocentesis is important,  as aspirating synovial fluid into the syringe further confirms true intra-articular positioning of the needle tip, and complete decompression of the joint by arthrocentesis prior to injection increases effective intra-articular concentrations of the injected drug, improving clinical outcomes. Thus, present studies further emphasizes the need for complete arthrocentesis and decompression of the joint prior to injection of intra-articular medications.

References:

*Importance of synovial fluid aspiration when injecting intra-articular corticosteroids. Weitoft T, Uddenfeldt P. Ann Rheum Dis 2000;59:233–5.
*Chronic knee effusions in patients with advanced osteoarthritis: implications for functional outcome of viscosupplementation. Waddell DD, Marino AA. J Knee Surg 2007;20:181–4.

Ultrasound used for incresing accuracy of injection comes with several drawbacks:

It takes months and in some cases years to master the technique
Sometimes requires an assistant
Maybe another source of contamination
Still not 100% accurate
Costly