Revenue recognition The Company recognizes revenue when we have satisfied a performance obligation by transferring a promised good or service (that is an asset) to a customer. An asset is transferred when the customer obtains control of that asset. The following policies apply to our major categories of revenue transactions:  Revenue is recognized when product is shipped at which point the performance obligation is satisfied and the customer obtains control of the product.  We place certain of our capital equipment with customers on a loaned basis and at no charge in exchange for commitments to purchase related single-use products over time periods generally ranging from one to three years. In these circumstances, no revenue is recognized upon capital equipment shipment as the equipment is loaned and subject to return if certain minimum single-use purchases are not met. Revenue is recognized upon the sale and shipment of the related single-use products. The cost of the equipment is amortized over its estimated useful life which is generally five years.  We recognize revenues in accordance with the terms of our agreement with MTF on a net basis as our role is that of an agent earning a commission or fee. MTF is responsible for the sourcing, processing and distribution of allograft tissue for sports medicine procedures while the Company represents, markets and promotes MTFs sports medicine allograft tissues to customers. The Company is paid a Fee by MTF which is calculated as a percentage of the net amounts invoiced by MTF to customers for sports medicine allograft tissues. The Company accounts for the services provided to MTF as a series of distinct performance obligations and each service is recognized over time as MTF simultaneously receives and consumes the benefit.  Product returns are only accepted at the discretion of the Company and in accordance with our Returned Goods Policy. Historically, the level of product returns has not been significant. We accrue for sales returns, rebates and allowances based upon an analysis of historical customer returns and credits, rebates, discounts and current market conditions.  Our terms of sale to customers generally do not include any obligations to perform future services. Limited warranties are provided for capital equipment sales and provisions for warranty are provided at the time of product sale based upon an analysis of historical data.  Amounts billed to customers related to shipping and handling have been included in net sales. Shipping and handling costs included in selling and administrative expense were $14.0 million , $13.1 million and $13.4 million for 2018 , 2017 and 2016 , respectively.  We sell to a diversified base of customers around the world and, therefore, believe there is no material concentration of credit risk.  We assess the risk of loss on accounts receivable and adjust the allowance for doubtful accounts based on this risk assessment. Historically, losses on accounts receivable have not been material. Management believes that the allowance for doubtful accounts is adequate to provide for probable losses resulting from accounts receivable.  We sell extended warranties to customers that are typically for a period of one to three years. The related revenue is recorded as a contract liability and recognized over the life of the contract on a straight-line basis, which is reflective of our obligation to stand ready to provide repair services.